I've found that if there is open F then I will have the option in the app to confirm the new flight in the upgraded cabin. If there is no F available, I can select other seats and will get in Y and then be back on the upgrade queue like normal. I could be wrong because I book paid F about 50% of the time anyway...but I'm almost certain that is the case. If anything, it is easy to test and verify without making any changes.