This has happened to me twice in the past and both times it was with the now defunct company Golden Sun. On the last occasion i was furious because it was less than 2 weeks before i was due to travel and i already had my tickets. I got a phone call from my travel agent informing me of the change.
I spoke to the tour operators direct and was informed that it was due to operational changes and that they could do anything they wanted as long as i was put in a hotel of at least the same class.
I was good friends with the owner of the hotel i had origionally booked into as we had stayed there twice before. She told me that the tour operators had wanted her to take less money...............7 euros per night per room, down from the 15 euros per night that they had paid her the year before. They also wanted her to reduce the quality of breakfast from a buffet breakfast to bread rolls and coffee and also change the bed linen twice a week instead of 4 times a week.
She refused to do this and kicked the operators out of the hotel saying she could not operate on that amount of money.
I think you will probably find that this may well be the same sort of reason your accommodation has been changed.
Contracts in general are drawn up long before the seasons commence but the tour operaotrs are well known for going in at the last minute and trying to negotiate lower prices, often trying to scare the owners into accepting in case their accommodation is left empty if the operators "pull the plug"

John.