If that is the reason for the change I don't think it will help you unless you do the following:
Only the primary account holder's MyBT BTID will have access to BTSport.
This means that your new primary account will till have access to any of your email addresses unless they have been gifted. This will mean that you will have to gift your present primary email account to your self to avoid the same problem you presently have.
That may be what you want but keep in mind that the primary account MyBT BTID also has access to all your billing details and can also make or delete any of the sub-accounts.
That's fine. I am happy either way. Either, if it was possible to have a completely separate log in for BT Sport or for the current Primary email account to be changed and removed from the my BT log in. I don't have an issue with people being able to log into BT Sport and My BT as long as they do not have access to the email address which is currently the primary email address.
As I have not heard from the moderator's I assume they were unable to help on this occasion, but thanks for trying anyway. I will go back to BT and see if I get any further with them.
You will need to gift the email address that you don't want others to have access to so it's separate from the main BT ID with sport access. This email guide will show you what you need to do to remove it from your main BT ID details.
I hope it gets sorted but if you need any further help send over your details using the contact the mods link?