Thanks. Good points also and something to keep in mind.
I should share a bit more info though, just so people are aware of the situation with this particular cluster. We have 15 or so clusters around the world and some of those regions have more physical phones than others. Some regions have more people in the office than others tend to have. This cluster I'm dealing with is one of two in the US and things have changed quite a bit ever since the 'lockdowns' a few years ago. More and more, our clusters are becoming more of a call routing hub than a set of servers the devices for people and room devices register to. Some of them have always been heavy on the call routing, especially the ones that route calls to Teams and our inContact call center. Out of a little over 3,000 numbers in use on this cluster, there are only 370 physical phones and of those, 206 are common area phones like conference rooms, 54 out of the 370 are unregistered for various reasons (office remodels, etc) and just 110 actually registered and with a person's name assigned to it. Out of those 110, I would bet most of them rarely come to the office anymore. Everyone that has a phone has a remote dest profile with a remote destination setup to ring MS Teams simultaneously. Hardly anyone would have access to actually make changes to their phone, like changing forwarding or speed dials. We have Extension Mobility enabled, but only a handful used it in the past (those people no longer have phones). We don't have Expressway anymore, so no devices are registered offsite. These are all in-office phones I'm talking about. We only have a few special cases using IP Communicator and we've already decommissioned Jabber (like I said, MS Teams is the primary telephony tool for our users now (especially in the US), so there's just not much user activity on CUCM related to anything a user could possibly change. We had CMS (Cisco Meeting Server - video conf), but that was also decommissioned in favor or Teams/Microsoft Teams Room devices in conf rooms. Most of any type of changes made on CUCM are only made by us, the admins...things like adding a new CTI Port for a new user that's permanently forwarded to Teams or removing something like that for a terminated employee.
So, with that being said, maybe you can see why I'm not too concerned about losing any changes people make because hardly anyone has the ability to change anything (speed dials, forwarding, etc) and of the small amount of physical phones left, most people would never forward their number because they have a remote dest profile that's already ringing their Teams client.
Thanks for the advice and notes about those things I didn't think of. Mostly I just wanted to be sure there was no technical reason why I couldn't upgrade and then switch a few days later (just to make my weekend work shorter and leave more weekend time to troubleshoot problems, if I find any). So, if I'm understanding correctly, there's no technical reason why I couldn't do it this was, but yes...I'd have to keep in mind the things you mentioned which could happen in a few cases, but I think those would be rare cases, given how we use our cluster and with how few people/users have access to actually change something on their end.
