anyone know if the later rotor is compatible across the earlier models?

rotor as in goats as in magnets on the flywheel. rather than brake discs.

All year rotors are compatible. I have a 1989 generator rotor on my 2006 GSF. The earlier rotors have fewer magnets. I don't know if one version is more reliable than another.
We are pretty sure the GS450 rotors are compatible too but I haven't seen anyone actually try a 450 rotor in a 500.

There are differences between years, check the part numbers.
As well as the number of magnets the length of them also differs by a few mm. However the diameters and tapers have not changed and they are all interchangeable. I have changed a few and not seen any significant change in how they perform 

If you are going down the used part road make sure the taper is in perfect condition. If you see any sign of damage or scoring forget it. It will not grip the crank and will ultimately loosen off in use. I would also suggest you lightly lap the tapers together with very fine valve grinding paste