From a theoretical perspective, I really like the idea of just replacing the panel guts and cover- especially when the old panel is recessed and piped in. Yet, I have yet to do it this "easy" way.
Why? Because the cost of the one retrofit kit I priced was at least five times the cost of a new panel.
You would think that someone would make their panels to the same overall dimensions as someone else ... but it appears that a deliberate effort has been made to ensure that today's cover never fits yesterday's panel.
Schneider owns both Sq D and the current version of FPE (in Canada). One would think that Sq D would market interchange kits here, to facilitate replacing FPE panels here, using Sq D products .... but such is not the case.
Heck, I have a damaged GE panel that needs new guts ... and I've yet to find any GE resource that will identify the GE guts I need to get to replace the existing ones!
The panel makers certainly have the information necessary to allow for the replacement of panel interiors, but have mainly chosen to play 'dumb.'