Is it possible to switch out the multispectral sensor with the standard 20MP camera, in order to utilize a single platform for multiple mission types?  It seems it may be as simple as swapping the gimbal unit, but I would assume there is also a firmware change needed.  Any info on this particular matter concerning the older phantoms would also be appreciated.  Thanks.

They're certainly not designed to be swapped by the user... Not saying you couldn't but it doesn't seem to be something you'd want to do on a regular basis. Too much chance to gank something, and as you said there may be FW or other changes that must be made as well.
The Phantom platform is simply not intended for swappable payloads. Get a Matrice.

I do have several p4p's, and want to swap the multispectral camera on one of them with a standard camera. (Because of the rtk-module on the multispectral). Is it possible?

Nothing.
Good to learn what to expect from the official dji forum

Lol. You know without asking what the "Official" DJI response was going to be, if they even responded at all. "Not possible. Not supported. Not recommended".

Not sure what you expected.