[morzh]

It is simple.

As for "can they remain in place" - well, if this means "can I simply parallel new caps to the old one, using it as a solder post" - no, you cannot. The old cap, first of, retain some capacitance, often it goes up; what's worse, it develops leakage (which is the reason it gets replaced) and this leakage will be in parallel to the new cap. So you will end up with not cured leakage problem and wrong capacitance value.

However, if you use it purely as solder post, making sure the old capacitor does not become part of the circuit, then yes, ir can stay. I see little reason if any to do this, it is wasteful and, to me, simply counterproductive, but I've seen this (and invariably would do away with it).