Should I replace capacitors in an old but working amp?

Absolutely. You have a 30 year amp, and it's' time.

But, modern caps last longer, and if you get higher temp variants they last even longer. So go ahead and replace them and you'll be good for 20 more.

The one downside is that you won't recoup this cost. If you have been on the shelf of trading it in or not you probably shouldn't.

I enjoyed that amp in the 90's!  It's well worth keeping (even though I didn't....).  Yes, replace the electrolytic capacitors after 20+ years in any amp.  I did that with a tube amp and it was a very nice improvement.
If you are handy with a soldering iron, you can do it yourself for low dollars (actually, you don't have to be that handy, because I was able to do it - - just get a good Weller variable control solder station and a good solder sucker, and you can tackle anything).  There are many great Panasonic electrolytics that are readily available from Mouser or Digikey, and they are rated for higher temps.
You will hear a bit of "sonic restoration", I'm guessing.

Just replace the electrolytic power supply caps first after 20 years.

Film, mica, and non electrolytic capacitors can last for “almost forever”and I would not replace them.

If the equipment sounds like new, it is not time to do any repairs.

The value of any electrical component can sway over time. Focus on resistor upgrades as well. If chokes and transformers have values written on them then have them checked too.

The tonality may change in varying levels for every item that you swap. Try changing out resistors and caps of similar composition to retain as much of the amp's current tonality as possible. I know from experience.