manager changing usage weeks

This year, yet again, CGH have shifted around the high and medium season weeks to suit them. When we bought our apartment in the Alps it was based on Easter being a combination of medium and low season weeks so we were able to go for 2 weeks at Easter.Last year they made Easter 2 high season weeks. Now they have changed one of the medium season weeks in the summer to high season. These changes affect the economic proposition because if we can't use it when we thought we could it doesn't make sense financially. Our residence association is up in arms about it (English and French). Has anyone else experienced similar problems and any suggestions for how to deal with this?