A radical method ...
Disable the management port of the primary WLC. This will cause your existing WAP to go to the secondary WLC (your objective).
Next, in CLI (of the secondary WLC), do the command "sh ap summary".
This will give you all the WAPs connected to the secondary WLC caused by the failure.
Copy the entire list and using cut-n-paste, you can generate a CLI to change the primary WLC details to a new one.