There have been many reports of CO making miles upgrades available via the F bucket but zeroing it out at the EUA runs. I haven't heard too many of them lately, but it sounds like that is what happened to you in this case. As for how quickly they can do it, my guess is that they have an option in the computers to do it so it is relatively instantaneous. What's the value of a computer if you can't leverage it for things like that.

By virtue of the fact that the other 3 exit row seats are still taken at least you know that someone wasn't upgraded over you.

In the mean time, CO will take miles, a Y fare or an F fare purchase and happily give away those seats. Or you'll get your seat at the gate. Or tomorrow. Or the next day. Or they'll sell out on higher fares and you'll keep your exit row seat. Hard to say, really.