Update: About as soon as the KB was released, a patch came to resolve the issue. Find it here https://support.microsoft.com/en-us/kb/3061059
For those of you getting ready to upgrade from Lync 2013 to go to Skype for Business, if you have Enterprise Voice you need to be aware of a known issue with Skype that could be potentially serious (but a workaround is available).
The issue can occur when you move your Exchange UM objects to a Skype for Business server, there may be transferring issues. There’s a discussion related to it here on the TechNet forums: https://social.technet.microsoft.com/Forums/office/en-US/d91960c6-6e3c-4000-9c11-da208d3d0225/warning-skype-for-business-server-2015-has-a-normalizationmatching-bug-problem-for-those-with?forum=lyncdeploy
And a KB article was just released by Microsoft: https://support.microsoft.com/en-us/kb/3069206
There is currently no fix, but the workaround is to leave these objects on a 2013 Front End pool. You can move pretty much everything else, but this might slow the decommission of old servers down. The fix is expected soon, and will be one of the first released if not in HF1. In the meantime, there’s no reason not to upgrade, there’s just a reason not to decommission 2013.
So far I’m not seeing many other issues worth chatting about as long as you’re following the upgrade procedures properly. Thanks and happy upgrades!