Account groups error

I setup a new Bill To and a Payer, I used the account group BILLTO Instead of PAYER, went back to XD07 to change the account group, then i recieved the message saying " The planned change is not allowed as the following field groups would be masked by the new account group." How to change the acount group now? Plz let me know with the steps to resolve this.

Thanks in advance.