It's hard to say. As you said, until recently, UA
required you to have 500-Mile Upgrade Certificates Domestically. They have a very large Premier base in Boston. I have pass benefits on UA
through my spouse who still works for them, it used to be very easy to get first non-reving. I flew IAD
on a 777 the week they introduced unlimited free upgrades, the night before I checked the loads First had 8/12 seats open, business had 25/49 seats open, and Economy had 95/197 seats open. We get to IAD
and First and Business were completely full! AA
though, has a much more sizeable operation in BOS
does, so they may have more AAdvantage Silver/Gold/Platinum flyers hungry for those F seats. Sorry for the long winded vague answer.
On a similar note, CO
is horrible to get upgraded on. Every flight i've tried to non-rev on, the list was about 20 people deep. The merger may mean more loooooong upgrade list, also keep in mind you will have the 1K's, and Global Services ahead of you on the upgrade list.