The best way to do this is to upgrade both controllers to the same code and then make them Master(M3)/Stanby Master(7200) using VRRP, this will make sure that all your current config is shared to the new controller
Make the VRRP priorty for the M3 higher so the APs are still seeing that controller as the master .
Then when you are ready to make the switch change the VRRP priorty on the 7200 to be higher
#7220