when do you replace capacitors?

Do ALL the caps at one time when you decide to have them done.
Twenty years is a safe period.
It might not be neccessary to have all of them done then, but better that, if you really like the amp, then have one blow up on you suddenly.
I you want to do them early, no problem. Especially if the company is still able to do it for you, instead of some third party.
Think of it this way: if you do them now, that gives you 20 MORE years of service from now. And in four years, heaven forbid, maybe they will not be able to redo them for you.

Elizabeth,
Does a stored or "retired" component experience aging of the capacitors and resistors at the same rate of speed as a component actually being used over the years?

Thanks. Will plan on doing it before long. Naturally, I want better capacitors than the manufacturer would use, at least in some spots, so a third party will be needed.

As for a long time use/non use, I would say the time is time. used or not, for caps. Capacitors actually LIKE to beused some. Not too hot, not too seldom.
So the twenty years is used a lot or not much.
Some discussion is given to theory about high temps will cause premature aging of caps in hot running electronics, and that they dry out faster. But little real evidence to show that is really true IMO.
Some caps just leak or die no matter what.
The one bad thing is a LONG time and never used, is very bad for capacitors when they get turned back on for the first time. KaBOOM!
A super no cost way to reform them (what the process of getting them back after yearss of no use): plug the unit in. Turn on for the briefest possible time fast switch on/ off under one second. Wait ten minutes. Turn on again for only two seconds or so, then off, (don't stretch out the time) leave off for one half hour. Turn on for five seconds, then off. leave off for several hours to overnight. Then it should be good to go.
Cheaper than other ways to safely reform caps. Works too.

Elizabeth,
Thank you for your reply.
Seeing your name on a posting here on Audiogon is cause enough to check out the words of wisdom and experienced opinion to be discovered.