I'm on the subscription Pro license for Unity 4 with installs on my MacBook Air and iMac.
Currently, it appears I have to de-license Unity 4 to be able to use Unity 5 as a pro user. (technically I can't even keep 4 licensed and use 5 as personal on one machine due to the EULA). This makes it a complete pain to see if our current projects will work properly in Unity 5 and is a major faff when it comes to finally switching to Unity 5.
The effect is I'm not rushing to upgrade to Unity 5 as I've got projects underway that I don't want to risk breaking or losing time on.
What I'd like is for the subscription license to properly be per-seat. So that whatever version of Unity I launch on that machine will recognise its license and make transition between versions a lot more practical.