I just did this too by dropping a line to You First and it was no problem. Needed two middle pairs on an A380 to Vancouver as 2 adults plus 2 kids and our usual row 2 was taken which left rows 3 and 4 with the blocked bassinet seat. That said, I'm secretly hoping that there is a spare window seat on the outbound as I'd love to spend some time looking out over Greenland and northern Canada.