This actually happens quite often, especially to people who book a year or more in advance. Ships will be repositioned or a private group will charter the entire ship and those already booked have their cruises canceled.
Depending on how close to the sail date your are you may get some additional compensation other than just your deposit returned. If it's a year or so off you usually don't get anything else.
I'm glad you were able to find another cruise that meets your needs. I'll be on Liberty this weekend and we'll try to leave it intact for you.