I have changed seats after upgrades have been cleared. Also, I regularly pull up the seat map after I have boarded (usually when i have an empty seat next to me and I want to see if it's going to stay that way); but like you said, it's read-only at that point.

FWIW, I expect GAs to be much more stingy with gate assigning C+ for non status pax now that there is an added cost for DL.

Also, (and while I hate to give away the secret I think it has already been mentioned several times) anyone who wants a C+ seat need only check in exactly at T-24. Every aircraft type has between 2 and 6 C+ seats held back until check-in begins.