The problen with 3.5 is they re-issued the books so it quacked like a different game and once you had the 3.5 books you no longer refered to the old ones and they compounded this by re-issuing all the splat books as well.The thing is in 3.5 the "core" rules didn't really change either. The backbone of the system was still there, sure they changed a few things and added some.
It's like taking a standard Chevy S-10 pick up, adding a lift kit, big off-road tires, a roll cage, lights on top, a new paint job etc.. It is essentially still an S-10 pick up underneath.
That is where you chevy analogy breaks down. If you were replacing the gearbox of the modified Chevy you still be buying the same parts as a standard model but with 3.5 they may have been the same parts but they came in different packaging and with different model numbers.