Balance Billing for Invisalign Case?

My case is $5500. My father's ins. was preauthorized to cover 2000, my mother's was 1000. I am making payments on Care Credit for the leftover $2500. Fast-forward 1 year (2013), my dad changes ins. comp. and I am 4 months away from the end of my treatment. Office said I have to pay the balance of 850 because my fathers original ins. won't cover it anymore even though it was preauthorized in 2012. Do I have any options here? I cant afford to pay more than what I originally was told to pay.