The 89-90 hub does not have a machined lip to support the center of the brake rotor. The 91+ hubs have a machined lip to help center the rotor that fits a larger opening on the brake rotor. That means a 89-90 rotor won't fit on a 91+ hub, but a 91+ rotor will fit a 89-90 hub but won't gain the support of that machined lip.
A replacement 89-90 Hub is around $200. A replacement 91+ hub is about $75.
So the story here is you can retain your 89-90 hub if it's in good shape. But when it's time to replace it, move to the 91+ and get some new rotors. And if you're getting new rotors now, get the 91+ rotors. (Any of the upgraded brake rotors should be for a 93+ already)