Delays / Not honouring pre-booked seats

We were delayed 2 hours going out and 30 min coming back and to top it all - we pre-booked seats, checked in online and then when printing our boarding pass we noticed our seats had not been honoured. When I spoke to one of the ground agents, I was told, sorry you can claim a refund on your return. This is not acceptable so I marched off to the ticket office and would not move until they were changed back. An aircraft change was blamed - sorry but what has this got to do with it - the same seat configuration was on the new plane so this is not an excuse. We were not the only ones affected - there were many upset passengers. Not acceptable!