The Mutant Earth will stick with the 3 original D&D classes (almost totally because I hate the personality of thieves, not because of the mechanics. Ain't that silly?), but I'm going to need another one to cover all this new technology in my game world--The Technician.
My first plan was to give The Technician a growing number of dice to roll to reach a target indicated by a piece of technology's Tech Level, modified by alieness. But that means I'd have to give every piece of tech its own score. The hell with that. I think I'll go with something like the Thief's straight percentage chance to succeed (though with target numbers instead)--success in this case indicating that the Technician can operate the new tech. Prismatic Death Ray, Matter Transporter, Space-Bender Drive, or what have you.
I'm also considering chucking the Vancian spell system for Magic Users and replacing it with something less bound to books, though I'm guessing it will still be firmly rooted in European folklore.
Clerics need to become less Christian--and since I imagine Space Fantasy games as godless (except for demons and Great Old Ones, of course), maybe the class will deal with spirits, energies and ghosts instead of the Divine.
And Fighting-Men are pretty much the same wherever you go in this vast, cold universe. They just have rayguns sometimes.