Psych Billing 90809-90807

I have a Psych MD that bills UBH a 90809. UBH denied the claim stating we need to bill at Contracted CPT code. According to UBH they will not pay higher then a 90807, and we can not change the contract. My question is... Is it legal to change bills from a 90809 to a 90807 to receive payment from UBH? Claim have been previsouly submitted as 90809.

thank you
Jen