I think only a few pairs of UD seats are available to pay to reserve unless you're SCH or higher, in which case you can reserve them any time. When I was Bronze, I had the ability to select 62A/B or 64A/B on one of my flights at T-7. The flight didn't fill up, and the UD was less than 3/4 full and I had a great trip with excellent service.

The chances to fly on the 744 UD are becoming fewer. So, if you've ever wanted to try it, now is your chance.