Your "old" Rotel shoud be fine. For amps to begin to age, I think they generally have to have long long periods of inactivity (the capacitors will begin to equilize), and of course, the amps have to be pretty old themselves for the caps to not recover from just repowering up. I have only experienced this with some of my vintage gear, and recap'ing them isn't very expensive. Some of my gear from the 70's that I don't use anymore I leave turned on for a few hours every once in a while to help keep the caps up. I could be crazy for doing this (if so, I'll add it to the list), but it seems logical. My Parasound amp from 1989 or 90 still sounds fine too. If you were going to get a new amp, you don't need to do it just to get a newer amp. Like mike c said, if you were to upgrade, do it if you need more power.
Brad