Yeah, upgrading from what speed? if you are upgrading to 8 speed from 7, you won't get much of an upgrade for the time and money involved. Just not worth it unless your current parts are in poor shape.

When you upgrade to a new speed, you will need a new wheel with a modern 8-9-10 speed freehub, a new 8 speed cassette, new shifters (8 speed), and at that point, might as well buy a new chain.

If your current rear speed is a 7 speed, the most likely your current rear derailleur should work fine. If that one doesn't work, you will then have to buy a new rear derailleur as well but that usually isn't a requirement. However if the rear derailleur is a cheap claw style SIS derailleur, then that is another flag that your bike isn't a hardrock. I think that most Harrdrocks came with frame mounted rear derailleur hangers and would not use a cheap claw style derailleur.