You can't unencrypt the password, but you can recover the password by clearing it when the WLC first boots (if it runs code 5.1 or newer). You have to console to the WLC via the console port and issue the Restore-Password command at the login prompt. You will then be able to enter a new password.
The Restore-password after the rebooting the controller sounds the best way. Do I have to worry about the configuration in the controller. This is a primary controller for the environment and I am not sure if the config file that is backed up is the recent.
This might help...
http://www.cisco.com/c/en/us/support/docs/wireless/2000-series-wireless-lan-controllers/71094-wlcm-pwr.html