As far as I understand the OP, the new seat is now on the other side of the plane. So probably the OP selected an A or F seat and now as an F (instead of A) or A (instead of F) seat, respectively.

I can understand that on some flights, one side is more favorable, for example if one wants to watch sunrise or sunset or in general want to avoid having the sun on the 'wrong' side of the plane.