5F and 5B are open if you move to them versus 4B or 4F. Though highly likely that 5D/4D would move to 5B/4B if you asked (as clearly not sitting with someone). As noted, I'd bet the gate agent would call that person up prior to boarding to ask for the switch.
The folks in 2F and 3A will already be attempting a switch in their seats.
But with 18/20 booked, and nobody on the upgrade list, you won't have an issue.