I have a 1994 Trek 8000, and I would like to upgrade it to a newer 9 speed groupset. If I find a used groupset will the cassette work with the wheels that are on my bike now or do you need wider hubs and a wider frame to fit it like when upgrading vintage road bikes?

Thank you

That's probably 7 speed cassette, huh? You'll need to switch out the hub for one compatible for 8-9-10. Shimano Deore can be had for a little over $30. You'll no doubt need new spokes. And, a twenty year old MB rim? May or may not worth it or even be reusable. If you're near the city (PDX) drop by Performance Bikes. I see some good deals on complete wheels there from time to time, even though they aren't on my "favorites" list. Check your LBS too, or even Craigslist for a good used wheel.

Sounds like I just need to order the hub then and a bike shop can get me all set up. Thank you!! My rims are one of my favorite things on my bike I hope I can keep them!

If your bike has a 7 speed drive train, and if your frame is aluminum, you might run into trouble fitting a 135mm hub in there without damaging your frame. Probably someone with more experience than I have will chime in and confirm that, but I'd check into that before I spent any money on a new drive train.

MTB Hubs were 135mm well before 1994, so that isn't an issue; but with 142.5mm coming in as the new standard, it will be in a few years.

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.

For your rims, you may like these, but remember, they are consumables, they will wear out, don't keep them on after they have worn too much.

For all the cost involved tho, if you have lived with 7 speed for all these years, will you benifit much from upgrading?

They look pretty good un-modified as can be seen here https://www.retrobike.co.uk/forum/viewtopic.php?t=70686

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.