Welcome onboard
As seating is pre-assigned, there's no chance of getting a window seat at check-in. Seating will have been assigned the day before even if you have'nt pre-selected a specific seat. As Sunaddict mentions, window seats are popular and there are only 16 window seats in the premium cabin, 8 each side so they fill up quickly. Aircraft changes only occur when there is a technical problem with the assigned aircraft or it goes in for maintenance (changes are normally done in advance and passengers told in this instance). It's a question of would you prefer a potentially lengthy delay, ie a day lost of your holiday, or a small delay and standard seating? I know which i'd choose! Given all the non-premium / smaller premium cabin aircraft are being used elsewhere at the moment though, it's unlikely you'll have a changed aircraft anyway. You'll find extensive aircraft information on our What Aircraft? guide (link below).
Darren


