I did this with my Mark IV a couple of years ago. (which is the same size cab) I purchased a new head shell from Mesa. That set me back about $275.00. Be sure when you order the cab to have them include a mounting plate & mounting hardware for your reverb. The reverb hangs upside down through a cutout in the bottom of the headshell.
Other than that, just unplug everything, take out the screws and move the chassis into it's new home. Reconnect the reverb and speaker and start playing. (Be careful DON'T Touch anything inside the chassis! It has enough voltage to KILL you even when unplugged.)
FYI - If you like build projects, I took the EVM out of my combo and built a homemade Thiele style speaker cab. Well worth the effort. If you are looking for more low-end punch (like a 4x12) this will do it.