Not difficult at all, the PS32 board with 5V connectors is again in stock at MTH. You simply have to remove the old 5V board, secure the new PS32 board, and plug in the connectors. One change is the speaker, you have to replace the old 16 ohm speaker with a 4 ohm speaker. The PS32 board is currently only available to ASC's, but you can obviously buy one from one of them if you need one.
The only fly in the ointment is occasionally the space available and the length of the harnesses in some diesels require some modification for the board to fit. Other models like the P5 or Bi-Polar are a very tight fit for the PS32 board and do require more work.
For that GP30, it looks like it would be a reasonably straight-forward fit, other than perhaps adjusting a harness or two. Since the big fat battery is no longer there, I think the board should fit. It could be close, the PS32 board is slightly longer... 
FWIW, run the 5V board until it croaks. Some of them run for a long time. 
