Changing contact information

I have just sold my house, which has a Tado set-up. I have successfully added the new owner to the home but I cannot change the contact information to his.

When I enter the email or phone number the error message says the email address is associated with another home. They have only registered with this home.

We can’t work out what the issue is or how to correct it. I can get any sort of response from the customer support team.