Not knowing the home builder and seeing the agreement you've signed, I don't have an answer for you - but will agree you have an excellent question.
If you're working with an agent, your agent should be asking for an explanation and documentation supporting the answer given.
If you don't have an agent, you'll have to ask and put the pieces together yourself. Actually, you should hire an attorney to put the pieces together (fact check) to put your mind at ease.
I can think of many reasons for a name change, but none of them are good.
Contact an attorney for advice.