As you opted to pay for a seat, the system won't now allow you to move to a "lesser" seat, and will instead only allow you to move to other seats of the same category.

Had you accepted a normal (no extra charge) seat, this would not have been the case - you would have been shown the entire cabin.

Usually, you can return to the online check-in page and change your seat; you can also do this on the kiosks at the airport, but the ability to view the seatmap (and therefore to change seats) sometimes is "lost" shortly before departure.