It is not very clear what you are asking.
Is this an email account that has been migrated from BTYahoo to BTMail. If it is your folders should have been migrated when the account was.
If you mean you have a BTYahoo email account and you now want to set up a BTMail account they will have to have different email addresses and one will not interfere with the other.
I have a bt email account from when it started with Yahoo. I’ve never bothered to update as it wasn’t necessary. I now deem it necessary and will do so as long as I don’t lose my material stored in files in my yahoo platform. Does this answer your question?
What do you mean you now deem it necessary that you update it and how do you propose to do that?
Is it a Yahoo email account or a BTYahoo email account.
If it is a Yahoo email account it will remain as is and will not be deleted no matter how many other email accounts you set up regardless of which email supplier you use.
If it is a BTYahoo email account, you can tell by logging onto the email account and checking the logo. If you see BT and Yahoo it means it is a BTYahoo email account. The BTYahoo email accounts are being migrated to the BTMail system If your account has been migrated you will have been informed by email and as I previously stated all your folders will be migrated at the same time and your email address stays the same.
It doesn,t matter that you deem it necessary or not. It is BT who will automatically migrate your BTYahoo email account to the BTMail system at some point in the future. Once they have done that you will be notified by email.
If you use email clients or apps to access your BTYahoo email account you will need to check and if necessary change the settings as per these links.