Gate agents can move you wherever they want, once it goes to gate control. They have full control over the seats. They often move people around to accommodate families and groups.

I would hope that they would give priority for "better" seats to people on main fares over saver fares. But people on main fares can choose those seats whenever they want, so, by the time it goes to gate control, I wouldn't see any problem in moving you to any non-premium available seat. They are generally very helpful (although often quite busy on a full flight, so you may have to wait).