What is the process to upgrade the parts of vehicle from 3rd generation to 4th generation?
What parts? Body, suspension, engine, transmission? And what kind of car? Way too vague...
Different generations of car are usually a complete redesign. For instance, you cannot upgrade a C3 Corvette to a C4 no matter what you do.
@Gary I meant a complete redesign of a car where and how will we start redesinging. The basic process involved in general and what would the limitations in redesinging. Take for example Toyota camry
You want to design the next generation Camry? You would need several hundred million dollars and a team of hundreds of engineers and designers to design a modern car that can meet all the legal requirements. You might need to spend hundreds of thousands just to design the door handles. Yes really.
What you can do is conceptual drawings or computer renderings.