Panel replacement

[kwired]

120 amps even for a short time builds heat and over a period of time wear down the components.

That AC unit will still need same size branch circuit device whether it is in a 100 amp panel or a 600 amp panel. If you are really that concerned you maybe need a AC unit with VFD or other soft starting methods.

 

[Jon456]

I'd still confirm the loads are actually what they were calculated/designed to be. Who knows what modifications a homeowner could have done on their branch circuits, what additional loads (overloads) they may have added, or if the installed equipment (e.g., the air conditioner) is still operating within specs.