Any specific reason to move to 17.6.X? If you are planning to use a new feature introduced in 17.6.X then this fine, otherwise this was released just few days back and I wouldn't recommend running it on your production networks without testing it on a testbed.
My approach for this would be first do config comparison manually (notepad++ compare) to see whether the new firmware has enabled some new features, then do a radio active trace to check whether there is any abnormalities, then I would do a pcap from client side (if possible) and AP to see if this can give us any information.
Also note that RF behavior and some RF related new features were introduced in 17.4.X and higher. (OFDMA enhancements, scheduling enhancements, MU-MIMO enhancements) May be this will have an impact on the client behavior with the new IOS. So take this also in to consideration and read all the release notes.
