Having Trouble Transferring Billing Responsibility

My phone bill has been under my dad's name. I recently decided to make it my own. I got the link to accept the transfer, but when I put my phone number under "number you want to take ownership of" it won't let me keep it under the existing account. It will when I clicked for "new account" but I don't want a new account. How do I do this? I just want to change from my dad's information to mine.