When to get your amp up graded caps ???

I'm with Has2be, Wolf-Garcia and Liz.
Macs would less-likely need 'upgrade' or change for the better components which will probably be the waste of time and money.
If the time has come, you'll need to replace them with identical equivalents. To test caps(mainly DC) you'll need cap tester or DMM capable to measure capacitance in order to randomly pick up one of the crucial caps that you would like to compare to the rated values. Normally there's a lot of headroom for needed capacitance so if the DC caps loose their capacitance there's still a room to play more music before the replacement occurs. Hence you might be still OK to enjoy your amp.

I own unit Classe 30 preamp that is almost 30y.o.
It hasn't gone through any recap or repair process although I checked-measured filter caps and tey've only lost 3..4% of labeled capacitance. Having perfect pro skill and instruments to replace circuit elements I'd prefer to be resourceful and keep healthy components up until later and enjoy the music. Usually it might happen all of a sudden that they will dry out. Than this will be audiable.