Hi
you will see from the recent posts here that you are not alone, we have just been in a similar situation, however ours was a complete change of airline, where as yours I take it is a change of aircraft but same airline
In the first instance go in and see your travel agent to discuss your options, we were very lucky to have a customer care manager in store who didnt fob us off at the first hurdle, although we did have to go through 3 days worth of staff to finally get to speak with him directly
you will be told its within terms and conditions as we were, however keep at it as we did, and depending on when you booked and the circumstances around it you may just find an amicable agreement, above all though dont lose your cool as they will not respond to you helpfully
I work in customer care/memberships and its never nice to be on the receiving end of a heated complaint when its your employer that has put you in the firing line
Hope you get sorted
Karen