This is goofy but the enclosure and panel are two different items in MDPs. So you can swap the guts in a panel which is why you can buy a bare one to mount in say a prepared space in an MCC.
Repairing is usually replacing broken parts. Mostly field work.
Reconditioning is taking it completely apart and restoring to factory new. Weld up and drill and retap holes for instance. Replacing brackets and other parts, rewinding coils, repacking bearings, replaying, etc. Strictly shop work. UL Listed result as an example.
Retrofitting is when you remove the existing equipment and replace with new modern equivalents within the original assembly. This would be for instance keeping the trucks and frame of an old breaker and mounting a new vacuum breaker on the frame.
In terms of transfer switches it depends on the type. Some are really just reversing contactors wired as transfer switches. Some are basically double throw switches that are usually custom built. I could see easily retrofitting the first one because it’s essentially an industrial control panel subject to UL 809A. But the second is usually a throw away. In fact it’s rare they can even be repaired. I’ve seen guys try though, but the results are what you’d expect...garbage in, garbage out. The workmanlike installation rule comes to mind.
