The last time we did a CUCM upgrade (8.5 --> 8.6), the biggest thing that didn't go well was in regards to firmware. The vast majority of our phones (7962s, 7965s) stopped working properly. They handled calls fine, but the Corporate Directory and other services became unavailable.
We found that the only way we could fix it issue on an individual phone was to perform a factory reset and have the phone download it's firmware all over again. This solved the problem for most of the 300 or so phones, and a handful basically died completely, not being able to negotiate with the tftp server after the factory reset.
We're getting ready to upgrade to 9.1, and I'm curious as to what is the best practice regarding firmware and upgrades? We're currently running SCCP, but I think I want to convert everything to SIP, but that is over a year away. I hope to get the upgrade out of the way next month.
At present I only have 1 phone with a mismatched load on my Publisher and 16 on the Subscriber, and I thing the bulk of these are phones that never got resolved after the last upgrade
Thanks!