I also found this a bit frustrating. I even used two C2 behaviors that C3 also features; MoveTo and PinToImagePoint. Yet C3 wouldn't simply 'replace' them.
It appears C3 is programmed differently, hence we can't copy code back and forth.
And as you mentioned, I would definitely agree that importing a C2 regardless of missing plugins etc. would make life easier. They could just permanently 'toggle disabled' the code or something, so you could at least review it and replace it below those lines, and then delete the old code yourself.
Now I had to delete all the lines in C2, import, add the code again, add the behaviors, etc. while switching back and forth dozens of times between C2 and C3. Fortunately in my case it was only a bunch of lines, imagine wanting to import a huge C2 project..