For Geeps made during the MPC era, the GP-7's and 9's are easy for the most part.
That said, Mt. Clemens did change the handrails from a fragile (but more realistic)
wire handrail with plastic stantions to the PW style stamped metal rails.
Switching these shells will take a bit more effort, and the handrails are part of the frame. In the case of the wire handrails the plastic stantions are part of the frame, but can be removed.
The GP-20 shell (which was created buy inserting a new tool into the
GP 7/9 mold) will also take a bit more effort. To create the longer hood for the GP-20, the headlight for the long hood was modified, and uses a "christmas light" type bulb.
If you place a GP-20 shell on a GP 7/9 frame, and leave the 7/9 headlight "as is"
it can melt the top of the GP-20 long hood.
Most of these diagrams are in the files section of the MPC yahoo group.
Ken