vs044 seat prebooking

hi
we booked our seats back in June for our Vegas trip, out in Y and back in PE and were able at the time to select seats - great for the Y option as we got the ideal ones we wanted, however the PE ones we chose were upstairs in again our ideal slot and according to the booking summary on the VS website still allocated, HOWEVER if you try to look at those seats in detail on the VS site it states that the PE ones will be allocated at the time of checkin and not before.
Now obviously we would like those upstairs ones for all 4 of us but if they've changed the plane that would be understandibly possibly impossible.
I'm wondering why they have changed the VS044 so far in advance - flying beginning of March, been like this since late August?
we booked our seats back in June for our Vegas trip, out in Y and back in PE and were able at the time to select seats - great for the Y option as we got the ideal ones we wanted, however the PE ones we chose were upstairs in again our ideal slot and according to the booking summary on the VS website still allocated, HOWEVER if you try to look at those seats in detail on the VS site it states that the PE ones will be allocated at the time of checkin and not before.
Now obviously we would like those upstairs ones for all 4 of us but if they've changed the plane that would be understandibly possibly impossible.
I'm wondering why they have changed the VS044 so far in advance - flying beginning of March, been like this since late August?