Symptom: When trying to upgrade your 5508 Wireless LAN Controller from an older firmware version (18.104.22.168 in my case), you receive the following error:
Solution: When upgrading the firmware on the 5508, greater versions need to be applied incrementally. Stock 5508 WLCs appear to be shipping with Software Version 22.214.171.124, the following firmware versions should be applied to reach the latest and greatest versions.
I applied the following upgrades to reach higher versions:
6.0.x to 7.2.x ED
7.2.x ED to 7.x