xfinity mobile change in Account Settings to match another account so I can then Link them

I've had several chat agents with xfinity tell me they are able to change my mobile number for one of my accounts, to match the other account, so that i can link them (log in once with one email address and then switch between accounts).  I am not allowed to change my mobile number, because every time I try, the system says, "this phone number is already linked to another account."  So I can't change the mobile number, and then the chat agents assure me that they are able to.  The last chat agent said, it takes 3-4 hours.  However, the mobile number never actually gets changed. So i have to start over with a new chat agent.  I have spoken with over 6 chat agents, and tried calling (but can't ever get a human).  Please help me understand how I can change the mobile number in the one account to match the other, so that I can then link them.