What are odds of GUC clearing?

I’d like to use a choice benefit (all 4 of my GUC’s) for a round trip to Europe for my wife and I.

My goal is to go from Main Cabin to Premium Economy. There are all 10 or so Premium Economy Seats available on both legs.

The site says I can use them. But I don’t want to use my benefit and then find out that they don’t clear. I understand that i can always try again with another flight but this is our only international trip of the year. I don’t want the upgrade credits to go to waste.

What are the odds of the upgrade immediately clearing? I have diamond status but have never used upgrade certificates before. Never been diamond before and it’s all new to me.