And up until last week, I've been able to get a move up to MCE at the gate for no extra charge. Today, the agent told me the system now tries to collect $$. She showed me as she tried to move me from 14C to 8D a big notice on her screen requesting $$$. In my case, a 1000 mile mid-con trip was $43 additional for an MCE seat on an E175.
I've found after a few weeks that this is inconsistent. Asking to move up to even an exact same kind of coach seat, but just a little closer to the front, will still yield a charge by the gate agent in PHL. I've asked a few times and they all immediately, without even looking at my boarding pass, say there will be a charge. Then they look at the computer and quote the exact price.
At a non-hub AA station, the gate agents seem to do it in a matter of seconds without the system forcing them to do an add/collect. I suspect this practice will be come universal at some point.