backing out of a purchase
Most condo purchase agreements allow the builder (but not you) to assign the contract, so there is nothing you can do about that. If, however, there is a "material change" you are entitled to rescind.
Material change is defined as something that, had you known it from the start, you would not have made the purchase. It has to be of sufficient significance that the average person -- not just you -- would consider it important. For example, a variance of more than 5% in the square footage is a material change, but less is not. Changes to the amenities, landscaping, number of floors, units per floor and other things that do not affect your unit are generally not considered material.
It is not clear-cut. A change in the builder is probably not material but in some cases it could be.
Practically speaking, you can ask the builder. They count on you not being willing or able to fight them on their decision, so they will do what's best for them.