How many seats in F are remaining open on seat map and/or fare buckets on your target flight?

Have you looked at your itinerary on ua.com to see what the offers are to purchase up to F?

Overall, I think paying more with the hope that it'll result in higher priority and yield an UG that you otherwise would not get is a fools game with the policies and systems that the current regime has implemented. It's not something I'd do, plus, on top of that, you've got to deal with the frustration of dealing with agents who probably won't be accommodating to your request to change w/o change fee.

Do you have an EF alert on R space set?