1. apply the QoQ hotfix (email adobe for it or dig it out of the hotfix section of downloads page)
2. some CFinput stuff for autosuggests got killed off (we have some super old stuff in some places)
3. there are some issues with serializeJSON() converting yes/no to true/false that got fixed post 2016 that can trip things up
4. Dateformat issues, multiple workarounds -
https://helpx.adobe.com/coldfusion/kb/dateformat-function-coldfusion-2021.html
5. super random edge case that i am sure wont apply but we had to work around -
https://tracker.adobe.com/#/view/CF-4212005
We have not transferred all our servers over yet but the first two seem to have gone swimmingly once we fixed the above