One thing that I have done in this situation is disable adp on the new controller until everything is configured. This can be done in the CLI
(config) # adp discovery disable
This way, APs will not connect to the new controller. Once you have configured everything in the new controller, you can change your DNS setting (aruba-master) to point to the new controller and then run tthe command
(config) # adp discovery enable
At this point, things should transfer over. As mentioned, make sure that your authentication is working using the AAA test server (either in the GUI or CLI) before hand. Then test, test, test...
Just what I have done in the same situation. We have been known to change out the controller in our office often due to testing and have never had any issues doing this. Fortunately, in our sitation, we have a relatively small network. The only problem that I run into is with our RAPs. We do not use the self provisioning feature and, everytime that we change controllers, I have to reconfigure the RAPs (not a big deal for me however).