I just ran into the same thing and wasn't able to make a change to the redundancy after you set it... I basically factory reset and configured the WLC's again... this was easier for me than to try to mess around with it. Your probably going to have to visit that site to be honest.
For HA AP SSO, you should move from v7.4.110.0 to v7.4.121.0, which has been pretty stable compared to the version you are running. AP join is the same for HA, local mode, or FlexConnect. Here are some examples:
http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/70333-lap-registration.html
Thanks,
Scott
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-Scott
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