Originally Posted by
trvllvr123
I pre-assigned seat 64A for myself and my infant for our flight to MRU - Unfortunately we're now on a 777 so looks like we might be on the roof!

haha the same thing happened to me on a LHR-JFK booking. We're still booked in on 64A on the way back though, so yes you can pre-assign it.
Cheers, Simon