Where I see the log of update notes when tag templates are updated to idenitfy the changes to look for in upgrading from version x to version y?
I'm looking specifically for the Google Universal Analytics tag - although i would imagine other tags would be useful at some point as well.
usually on the top of the tag template you will see there is a description of what was changed - tc -, just after tv.
However, in order to see these comments, you will need to update the tag, what I understand you want to do just after checking what is going to be changed.
A solution is to add a new tag for the same vendor and, on Save/Publish, just SAVE it, you don't need to publish it to any environment. In this way you will be able to get the latest template and compare to your old one. You can delete this new tag after that.
Hope it helps. Let me know if you have more questions.