I am considering upgrading the Darkmoon fair trinket, but I see that the UI shows only the stats will be upgraded, will the use or proc be upgraded as well?
This site has a list of all potential gear and also shows the gear you currently have equipped on your character.
It also has a feature where you can upgrade gear and it shows the stat changes before and after the upgrade.
In this picture you can see that the Lessons of the Darkmaster Trinket has 847 Expertise and a Use that increases your strength by 4232 for 20 sec.
After upgrading it on the site, it changes to this-
It now has 913 Expertise and the use now increases your strength by 4561 for 20 sec.
This can be done with any piece of gear they have on there lists and is very helpful when trying to figure out what you may want to upgrade next.