Upsizing Capacitors

• ESR increases as electrolytics age, so renewing can have benefits. Measured C maybe well within spec, but ESR is likely off the chart on 20 year old caps.
• Capacitors are purpose designed, so ensure using the correct type.
• Replacing electrolytic and mylar with Teflon or Polypropylene is a sonic free lunch. Matching also pays dividends if the drivers are matched.
• ESR increases with voltage, so multiple parallel smaller caps can provide dividends. This is easier to implement in older / hand wired tube gear than modern SS or PCB tube.
• Take any rating by listening with a grain of salt. There are innumerable uncontrolled variables that come in to play when swapping LS XO capacitors and ALL impressions are only valid for that system only!
• ALL systems should have a soft start to reduce inrush current.
• Tube amps can benefit from a B+ soft start allowing heaters to warm before full B+ is applied.
• Fake capacitors abound, so buy from reputable sources like Mouser, Digikey, Parts Connexion, etc.

I would not change to higher capacitance and temperature, voltage maybe after looking the service manual. 

Why not? Some devices definitely have under temperature rating caps, particularly within a rack or under another component or heavy load or 24/7 operation.

105°C caps have 4x the life at rated temperature vs 85°C.

Modern caps pack more C in the same space, so increasing C and T while reducing ESR is a Win-Win-Win.

It depends where you are referring to . IHave been into  Loudspeaker mods for years ,and without question are upgrades in Sonic performance ,with Capacitors Resistors sometimes inductors, wiring, connectors.   For the vast majority of Loudspeakers use cheaper parts quality ,why cost. 
this is true also in electronics , types ,electrolytic in power supplies ,which should v be bypassed with a  decent quality .1uf  poly cap it smooths the upper frequency noise ,and quality here too very important like using a Elma Silmic cap much smoother ,or top Nichicon caps , even Clarity,or Mundorf 4 pole caps .

for coupling caps where a good poly cap, or top Copperfoil like World class Duelund ,which BTW I have been using for years I just exchanged the very hood Jupiter copper foil for the much better and muchBigger DuelundTinned Copper foil paper oil in my preamps output section , they take many hours to settle in but well worththe efforts.