Hi Hilton,
This is tricky and challenging using command "set scg ip <3.5.1 controller ip>" on the AP and deleting the APs from old controllers would work, if the AP numbers are more I would have those APs moved in one Zone and would run a AP-Cli script to do this, once the AP establishes connection to which ever controller it would try to upgrade the firmware that is available on the Zone.
I understand the method that you tried is harder.
Regards,
Abilash PR.