All the emails asking you to upgrade to new BT Yahoo will be fake. There is no need to upgrade anything.
When your btinternet.com/btopenworld.com email account is upgraded to the new BT Mail you will be re-directed to the bt.com website when you attempt to sign in via the BTYahoo! website. There is already a tab on the bt.com website for email users to log in whether or not their account has been regraded yet.
If you only use an email client (such as Microsoft Outlook for example) and never access the BTYahoo! webpage you will see no difference as the email settings will not be changing.
BT email accounts are being re-graded in batches and the changeover is due to be completed by the end of this year.
See this link on the bt.com website for more info
We're changing BT's portal and email services | Help | BT.com Help