Updating/backdating - I'd be ok with that.
For example, you'd be able to update an old BMW 3-series w/a new 3-series engine and you'd be able to backdate a new Honda Civic w/a 600cc carb'd motor. Stuff like that I'd be cool with.
When you get into cross pollination such as fitting a Subaru engine in a VW, then things can get out of hand and many hearts will be broken b/c a certain swap will not be available for a certain car.
In the real world, there's too much you can do if you're a good fabricator and all the possible permutations for 1000+ cars is really asking too much from a video game. Especially a 'simulator'.
When you have to start hacking/wacking subframes, change suspension points, wheelbases, track widths, and engine mounting points; you are, in essence, creating a whole new monster [for better or for worse]. It's too difficult to predict how that car will perform while it's too easy for bench/forum racers to say this & that.
Swaps across manufacturers will not be a good thing.
Now updating/backdating cars is something that is allowed in many forms of motorsport. I can see this working only if car classes are based on something simple, like power:weight + tire type & the physics is damn-near perfect. I say this b/c it may be easy for some 'swap' cars to slip through the cracks and become a very dominant car. That. Would. Suck.
Look at the AWD swaps in FM3; despite their added weight, they're dominant. Bad physics combined with a poor vehicle classification/PI system rendered many cars (90%?) uncompetitive despite what their 'stats' said.
If there cannot be parity & a reasonable 'class' system - I'd rather not be able to swap engines & drivetrains.