andymartin Last year our EDI-FNI flight (738) was cancelled due to French ATC strikes and we transferred onto EDI-MRS the next day (MAX) and our seating was now all over the place due to the row now being an exit row, and they ended up charging us extra for the exit row seats and for splitting us up!! At first the Ryanair online help didn't want to help but I wrote an email, explained exactly what had happened and they replied and refunded the fee for the exit row seats within a few days. I just methodically went through the issue and the consequences and they had no issue.
In your situation though, were your original seats in a different section of the aircraft? Ryanair don't charge less for the non-window seats than in the rows around them. In my situation I had no choice - the aircraft changed forced us into more expensive seats with no other choice. That may be the difference.