You have to realise that, although this forum is maintained by BT and overseen by BT moderators, it's essentially a customer to customer affair. If someone asks a question that other customers cannot answer it's often the case that the best advice they can get is to contact BT.
So you are not likely to get explanations of BT's operational policies here. You could try writing to them at
BT Correspondence Centre Providence Row Durham DH98 1BT
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Even though the last post stated contact BT, I did this 5 times and probably spent 4 hours on the phone. Each time I was promised the problem would be sorted, but these were false promises just to get me off the phone as the foreign operators seemed unable to help.
The final phone call instructed me to just create a new profile. This I attempted, but got stuck when I tried to enter the same original password.
It would appear new passwords must include letters, numbers and characters. I abandoned creating a new profile, returned to my original profile, edited my password to include letters, numbers and characters. I was then able to successfully change my BT user name email address and also change the contact email address.