The online seatmap is no indicator of how full a flight is. The economy cabin in particular will stir around right up until departure because of upgrade activity. You have a good chance of being reseated together. If it doesn't happen and you end up on board the aircraft in two different seats, you can ask others politely to switch, but always offer the switcher the more desirable seat, e.g. you must be willing to give up an aisle for a middle as your part of the deal.

I'm usually happy to entertain seat-switch requests unless (A) I end up in worse position or (B) the asker assumes too much, e.g. is already sitting in my seat when I arrive.