You will need to do the following :
- Make sure that both controllers are on the same AOS version
- Add all the necessary VLANs to the new controller and if not using Centralize licensing (AOS 6.3 and up) then apply the neccesary licenses to the new controller.
- Create a VRRP instance and point it to the Management VLAN (for both controllers) - It requires a reboot
- Point your APs to the VRRP Virtual IP
- Point your Locals to the new VRRP Virtual IP - Your local controllers will need to reboot after this change
- Configure Master Redudancy (On both controllers) , based on the priority set on the VRRP will determine which one will be select as the master - Make sure to set the priority higher on the current one