The present invention refers to a process for modifying structural members of vehicles subjected to electrophoresis treatments. It is known that the corrosion of metallic materials causes the generally slow deterioration of the material itself caused by the chemical-physical interaction with the surrounding environment. Due to the corrosive process, the metallic material is subjected to a progressive and irreversible decay, of a chemical nature, of its technologic properties. In particular, damages produced by corrosion are very relevant and are normally divided into: - direct damages related to the replacement of structures and components, due to product leakage, maintenance and restore, apparatus redundancy, design and insurance burdens; - indirect damages due to the reduction of the working life of plants and structures. As regards the provisions that can be adopted to counteract the corrosion, they deal with: ■ - design, with particular care to . the presence of interstices, water and humidity collecting recesses, vents, mechanical and thermal stresses; - material choice depending on environment aggressiveness; - choice of protecting methods that can be active when actions are aimed to reduce the electrochemical reactions speed, or passive when the action consists in the presence of a barrier that prevents aggressive species (water, oxygen, ions) to reach the metal surface. In particular, if we take into account the very frequent and highly interesting practical case of carbon steels exposed to the atmosphere, it can be observed that they are normally subjected to an attack that affects their whole metallic surface. In this case, a general corrosion occurs and the material progressively tends to be covered with a layer of non-protective corrosion products (rust) . Industrially, and above all in the motor vehicle field (but not only there, since . the following technologies and methods are currently used also, for example, for protecting high-range bicycle chassis) automatic methods adapted to protect the metallic parts subjected to corrosion have been adopted, both for reducing costs and for obtaining a greater quality uniformity and constancy. Above all in motor vehicles industry, methods based on electrophoresis deposition have been used for several years. In these processes, the metal to be protected, that can be an anode or a cathode of the deposition cell (anaphoresis, cataphoresis) is coated with a polymeric film that is formed after applying a potential difference between metal to be coated and a counter-electrode, immerses in a watery suspension of the product. The organic compound is partially solubilised (colloidal solution) in water, giving origin to micelles that are equipped with an electric charge that can be, according to the suspension, positive or negative. The application of the electric field generates the electrophoretic migration of the micelles, which, after having reached the metal surface, originate agglomeration phenomena, being deposited in the form of a compact film. The agglomeration occurs due to two different mechanisms: a) micelles lose their electric charge in contact with the metal that operates as electrode of the electrophoresis cell and therefore, not being subjected any more to the electric repulsive action due to the charge with equal sign, they mutually attract or collapse; b) the particles that are collected onto the surface tend to form secondary links one with the other and with' the metallic surface, such links favouring the constitution of a compact film. The process is very interesting from a practical point of view since it brings about the formation of a very uniform and compact thin film on the metal, reducing application costs and times. Initially, the deposit occurs on metal borders and edges, in the area where the electric field is stronger, but, since the polymeric film is insulating from the electric point of view, the deposition progressively moves till it affects all metallic surface areas, even the most hidden or unfavourable ones. It is also important to observe that the deposition process is very quick and the required currents are very low: in a few minutes a coating of 10 μm is obtained. At the beginning of this technology, an anodic process was used (anaphoresis), but afterwards a cathodic process (cataphoresis) was employed, that has great advantages: by adopting this latter method, in fact, the pH next to the metal to be coated is basic and not acid, thereby being less aggressive towards the metal itself. Consequently, the small metal dissolution, does not occur, that would bring about the incorporation of metal ions in the polymeric film, modifying their characteristics. Nowadays, therefore, the cathodic process is much more used, in particular in the motor vehicle industry. However, the particularly disadvantageous aspect, as regards the present invention, is the one deriving from possible mechanical workings of metallic parts that have already been subjected to an electrophoresis treatment (indifferently an anaphoretic or cataphoretic one) : in fact, in such situation, heat developed by typically used mechanical workings, such as cutting with rotating disks or traditional torch and welding for modifying one or more metallic elements, impairs the protection film and requires, at the end of working, a further application of the electrophoresis treatment for protecting the modified parts, this undoubtedly implying an extremely negative economic aspect. In particular, these considerations can be found in all those modifications that deal, for example, with mass produced, already finished motor vehicles regarding which every type of further mechanical intervention on structural members, such as chassis and/or body, would afterwards need a new electrophoresis bath. Object of the present invention is solving the above prior art problems by providing a process for modifying structural members of vehicles subjected to electrophoresis treatments that allows modifying, elongating, shortening such structural members without the need of employing a subsequent and further electrophoresis protecting treatment, thereby with high economic advantages. The above and other objects and advantages of the invention, which will appear from the following description, are reached with a process for modifying structural members of vehicles subjected to electrophoresis treatments as disclosed in claim 1. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims. The present invention will be better described by some preferred embodiments thereof, provided as a non- limiting example, with reference to the enclosed drawings, in which: - FIG. 1 shows a schematic representation of the steps of a variation of the process for modifying structural members of vehicles subjected to electrophoresis treatments according to the present invention; - FIG. 2 shows a schematic view of the steps of another variation of the process according to the present invention; and - FIG. 3 shows a possible realisation of a welding step of the process according to the present invention. The subject matter of the present invention is a process that allows modifying, elongating, shortening structural members of vehicles, such as, for example, chassis and body, subjected to electrophoresis treatments without the need of employing again the application of the electrophoresis, treatment for protecting the modified parts. Such procedure, in fact, in order to mechanically act on the metal parts, provides for the use of laser both for cutting and for welding, replacing the previously mentioned traditional mechanical workings. It can in fact be noted that the use of laser, both when cutting and when welding, does not impair the electrophoresis film, if not in a negligible way immediately next to the application area. Moreover, as regards the insertion and welding of new parts to already protected metal parts, the present invention provides for the use of stainless steel for such new parts in such a way that, as known, they also do not need further protections against corrosion. With reference to FIG. 1, it is possible to note that the process for modifying structural members of vehicles subjected to electrophoresis treatments according to the present invention provides, if such modification reals with an elongation of such structural members, the steps of: - cutting through laser FlOl a first structural member of a vehicle; - applying F102 a structural•modification member made of stainless steel to said first structural member; and - welding through laser said structural modification member to said first structural member. It is obvious that any industrial laser adapted for such purpose can be used with the process according to the present invention; in particular, a preferred laser is of the CO2 type; alternatively, another preferre'd laser is of the Md-YAG type. It is further obvious that such process allows modifying the structural members subjected to electrophoresis as previously stated without necessarily including the insertion of new members made of stainless steel; in fact, if a chassis or a body member had to be shortened, it would be enough to cut with laser the excess part and to mutually weld, always through laser, the two edges to be joined of the modified structural member. With reference to FIG. 2, it is possible to note that the process for modifying structural members of vehicles subjected to electrophoresis treatments according to the present invention provides that, if such modification deals with a shortening of such structural members, the steps of: - cutting through laser F201 a first structural member of a vehicle and removing a scrap member; and - welding F202 through laser edges of the first structural member to be joined. As regards the laser welding of the process according to the present invention, from FIG. 3 it is possible to note a preferred arrangement of the welding seams: in fact, in order to protect the minimum edge cutting area in which the cataphoresis coating has been possibly impaired, it is possible to employ a slight overlapping of edges respectively of the parts to be welded 20, 22 and to arrange the welding seams 24, 26 so that the cutting area is covered by the seams 24, 26 themselves; a preferable distance between the two seams 24, 26 is 30 mm, still more preferably 20 mm. Moreover, in order to further protect the welding area, it is possible to provide for the local application of a thermal spray made of aluminium. Moreover, it is important to note that the process according to the present invention obtains the previously mentioned advantages, whether the structural member has been subjected both to an anaphoresis treatment and to a cataphoresis treatment. It is further fundamentally important that the process according to the present invention allows, according to a non-limiting example, to be able to operate on a finished (new or used) motor vehicle to modify its structural members subjected to an electrophoresis treatment during manufacturing without the need of having to remove the parts that a further immersion in an electrophoresis tank would oblige to disassemble. This allows a high economy and an exceptional possibility of dividing the manufacture for all those steps that deal with assembling, disassembling and modifying vehicle refurbishing and aesthetic elements. Moreover, according to what has already been previously mentioned, the modification of vehicle structural members, by inserting new parts made of stainless steel and/or other adequate metals, has positive results on intrinsic vehicle safety since it highly increases its resistance to impacts.