That's good to know. I was a little concerned with the failover behavior of the APs when upgrading one WLC while both are "ON". So, I can keep them WLCs "ON" just modify the ACLs to deny any AP joins before the upgrade. Are there any other ways to make this upgrade as smooth as possible?

Also, the previous poster supplied a very helpful link.

Thanks again.

What I would do is set the primary wlc to one of the wlc's and then disable ap fallback. Upgrade the other wlc and when finished, change the primary to that wlc you upgraded. Only do around 6 ap's at a time and wait for them to upgrade the code, reboot and register to the upgraded wlc. Then when you have upgraded all the ap's, upgrade the second wlc that should have no ap's on it. Once that is upgraded and both wlcs have the same codw, thenn you can configure the correct primary and secondary wlc's. If you have WCS, then just make templates, so you can push that out.

I performed the WLC upgrade last night and all went well. I followed Cisco's recommeded sequence of steps: 1. backup the config 2. disable radios 3. upgrade 1 controller 4. upgrade the second controller5. renable radios

Thanks to all who replied.