That is a very onerous task you have in front of you. I wouldn't be able to go through everything and you might want to see out an partner's/integrator's help if you're not 100% sure you have a handle on it. My 2 cents:
* SPend most of your time making sure AND validating that any controller you are decommissioning, has the proper elements established on the new target controllers (VAPs, VPN Tunnels, etc, etc). The old adage of measure twice, cut once is more like 'Check configs 10 times, cut once'.
* Be generous with outages, as it may take longer than you think
* How are your APs provisioned/how do they find the master? If DHCP Opt 43, don't forget to change the IP, if DNS, don't forget to upddate the DNS servers.
* I don't have your network topology, but I would say get everything at the same code level, then slowly migrate groups to new controllers.
* Don't take any controllers down until you are fully migrated, in case you have to roll back
Good luck, be patient, take it in small steps if you can.