Have an email account to be retired, and wish to continue MS Teams (free, classic) as before with an existing MS Live account.
To sign in with a different account is as with a new user, and there are no options in the gui for resuming either current team activity, or activities for other accounts.
Right now, one way appears to be with using Exchange Online Remote PowerShell Module , undergoing deprecation, another way is to employ the more up-to-date Exchange Online PowerShell.
The best bet is a guide linked here (no gui), not sure if this works with MS Teams (free), see last comment.
Thanks.