Hi,
I believe the following is the correct procedure for this type of AP:
go to the WLC gui, and enter the following code:
config ap tftp-downgrade tftp-server-ip-address filename access-point-name
I would normally have suggested the 'standard' procedure, but this model of AP seems to require a different process, which is documented here:
https://supportforums.cisco.com/discussion/11774651/cisco-1552e-conversion-autonomous-mode
HTH
Mike