I am not going to search, but I think I recall reading on FF that revenue management can pull R availability just before the EUA is run, and reinstate it after the EUA. This allows passengers buying full Y fares (and B or M) to upgrade in the days before the flight. Otherwise, this option would disappear if there were enough high level elites on discount fares.

I don't like it because I almost never buy full fare and my upgrade percentage went down when this was started. I will say, that for whatever reason, the percentage has been better this year.

As many have said, if you must have an upgrade, then miles or money can make it happen. Otherwise, you get it when revenue management says so. I am at peace with that and appreciate every upgrade when it happens.