File opens in CS3 but not in CS4 (unexpected file format)

Hello! I’ve seen this with many .fla
I can’t open them in CS4, but they works fine in CS3 (so what I’m doing is opening them in CS3, save again, and then they works in CS4).
I think that is because the .fla are saved with Flash 8, but I don’t know why CS4 can’t open those .fla and CS3 yes.
Is this a bug?

Thanks !
E.-