Thanks, I forgot to post back that I got them off without any problem.
Getting them back on and tight is where I'm likely to do damage to the heads, or my new artwork
I did notice that the MDF holes are actually square, so I'm hoping that when the time comes to re-install everything, they will provide enough torque to tighten.
For anyone else attempting to do this, make sure you label your wires / microswitches. (I found it easier to remove the microswitch from the pushbutton, attach a small 'tag' with the button number on it).
The inside nuts on the pushbuttons are actually glued to the button itself. This is where nearly all of my frustration came in trying to loosen those nuts... Take a small screwdriver and just gently pry the nut away from the button until you hear the 'crack' of the glue breaking free. Then like RJPE mentioned, use a small set of pliers to loosen the nut while holding on to the button with your other hand.