thats all well an fine, however too many folks who are not technicians look at replacing a 20 year old cap as an "upgrade" when really its just simple maintainence that is required, not optional. This quickly could boil down to a discussion on symantics, but it is an important point for people to understand. Changing a dried out cap is like changing the oil on your car. I dont think many people would refer to an oil change as an upgrade and I dont think many people would resist an oil change because they dont want a fancy upgrade which they have been made to believe is snakeoil (and rightly so with the nonsense of hifi sometimes).
You can of course put very expensive components in your crossover, however, if you have 20 year old mylar caps in there and that bipolar electrolytic, putting cheap, fresh, new parts in there will improve the performance of your speaker. Not because you have upgraded it, but because its out of spec. Now, you can upgrade that electrolytic to a poly, you can change the iron core inductors for air core, you can do lots of stuff. But the simple act of putting fresh caps in an old crossover or amp is just flat out maintenence.