Can you be forced to switch to government lodging midway through TDY if a room become available later?

Need some input. I am going TDY and I, as a green suiter, realize that if government lodging on post is available, I accept. I input my dates and find that no rooms are available. I document this and reserve a room on the economy. The secretary then calls post lodging and finds out that if I change rooms midway that rooms are available and proceeds to inform me of this "change." I tell her that I have already documented the non-availability but she claims with this "new information" that I am required to accept on post lodging. This can not be true, can it? I can not find this specific circumstance in the JTR.