Upsizing Capacitors

You speak the truth! And thanks for that, great link - I'll have a read..

For tube amps, it is not a good idea to stray from the original capacitance values for the power supply. You can damage the amp. Higher voltage values are OK, though.

For solid state amps, things generally aren't quite as sensitive, but you can still quickly get to the point where the capacitance is a mismatch for the capability of the power transformer and rectifier circuit. 

A final note for capacitors in speaker crossovers -- changing the capacitance value here will affect the crossover frequency and thus the speaker's overall response. Presumably, you bought the speakers because you liked the way they sounded. You may or may not like the way the changes affect the sound. 

I've upsized large 4 filter caps on Accuphase E202 integrated amp on capacitance, temperature and voltage. The idea is to keep discharge time relatively similar. If the FACTOR of discharge time goes like 2...3x faster through the resistor, you'll need to replace it to the larger resistor. In my case the difference was tolerable only by 12% faster. Therefore I didn't change any other circuit components.

I have to assume we are talking about filter caps...

You have to be careful as bigger caps = more current draw and more stress on the transformer and upstream components.

Personally, reduced ESR and higher temperatures are a bigger factor in upgrading these than the capacitance. Higher temperature caps have longer endurance ratings generally speaking, so if I can fit a 105C cap in place of an 85C cap I will.

Best practices: If it works, leave it alone.