Originally Posted by
jimc101
MTB Hubs were 135mm well before 1994, so that isn't an issue
For upgrading to 9 speed, would have to ask why, 9 speed is as new parts go, is soon to be obsolete for all but the lower end groupsets, would be looking at 10 speed if doing any upgrading, if you don't have a 9 speed setup as a base, as it's the same cost as any 9 speed parts you will find.
For upgrading, you would need - hub, shifter, cassette, chain, F & R mechs, crank & BB, everything else on your bike should stil be ok. brake levers would only be an issue if integrated.
Assuming the frame is 135 mm, an "upgrade" to 9-speed would require a new freehub body, shifters (or integrated shifters/brake levers), cassette and chain and maybe a new front derailleur. The current rear derailleur and crank should work fine.
That said, the cost will still be substantial and the benefits minor.