A similar situation happened to me when I had booked a window seat on a 2x2 LOT Embraer 195, only to find a 3x3 B738 waiting at the departure gate. The LOT representatives were very much shrugging their shoulders, saying that my party got the seat numbers we had paid for. But aisle seats aligned with the wings offering no external views whatsoever wasn't quite the idea! We didn't pay a substantial premium to be lumbered with that. Online customer services were far more understanding of the issue and did refund the seat reservation fees afew weeks later.
In the above case, the switch from E195 to B738 was for the whole season. I reminded them that they really needed to inform all customers who had paid to reserve seats of this significant change. It is not unusual for airline executives to work on the presumption that people pay a premium purely to sit alongside their family / friends. There are other reasons.
When a carrier changes aircraft type without informing customers in advance, seat selection fees should be refunded automatically.