Great news.

The UI public source on bitbucket has now been updated to include a separate branch for the Unity 5 version of the source:

https://bitbucket.org/Unity-Technologies/ui/src/7c219d27562749b5581b017103c6947cebca6536/?at=5.0

There are several engine differences between the U4 and U5 UI source, so under the hood there are many changes and improvements.

However, your own UI source should still be 100% compatible between U4 and U5, so there isn’t any (much) impact on your own UI source.