Is it possible to check in online 24 hours before departure, and then change seat assignments after the boarding passes have been printed?

I've noticed that sometimes the best seat assignments come available a few hours before departure. However, it has been stated here several times that checking in on-line 24 hours ahead of departure can help one's chances of an upgrade at the gate.

So, can I have the best of both worlds - checking in early to get high on the upgrade list while being able to change seats in the coach cabin if perhaps an exit row aisle became available?

I am on a full flight to PDX next week, and right now first looks pretty full on that plane, too.

Thanks.