Thanks for the responses.
Here is my current scenario I was trying to figure out. I / better half are flying ANC - CUN next month on a companion ticket. I am currently in K fare.
I have had expert flyer alerts on this flight for months and no U space has opened up. There are between 6 and 9 open seats in F on most of the legs. My thinking was if I could upfare into a higher class ticket for a reasonable $ amount, its a gamble but more of an educated gamble. There is a chance with that many open F seats they might open up U space and if I am already in an instant upgrade class I will get the U space as it opens. Based on what feedback I have gotten it sounds like I should just keep waiting for U space to open and then do it.