Same thing has happened to us, our TA told us that Azamara had changed t&cs after we booked and we could no longer make one change (this we confirmed with agent prior to booking). We lost £1,500 as a result.
It makes no legal sense to me and, at the time, I intended to follow up, not being sure whether it was down to Azamara or incorrect advice from TA, or whether it's just because we are in the UK, it all seems very inconsistent.
One thing for sure is I won't be booking way in advance with them again, just pay & go from here in!