Premium Economy back from LAS

Hi,
I've booked PE seats on the way back from LAS in June - The only available seats were in the downstairs section of the plane, so we've had to take them.
However, PE tickets are still available, which leads me to believe that there are actually more seats available, up in the bubble - which is ideally where we want to go.
How do we go about finding out, and is it just a case of logging in exactly 24 hours before, and trying to move our seats around.
I've booked PE seats on the way back from LAS in June - The only available seats were in the downstairs section of the plane, so we've had to take them.
However, PE tickets are still available, which leads me to believe that there are actually more seats available, up in the bubble - which is ideally where we want to go.
How do we go about finding out, and is it just a case of logging in exactly 24 hours before, and trying to move our seats around.