Hi,
I think you should push back on Support to give you a definitive answer.
In lieu of that, and with a grain of salt…
Be careful not to remove the existing account prematurely.
Invite a new email address with any domain name other than your org’s domain (so it could be gmail.com) to create the new Guest login.
Login with the domain (Member) login. For every assigned task/subtask, change the assignee to the new Guest email. Same for all places where the Member is a collaborator. Account for all other ownership (like projects, portfolios, rules, custom fields if the Member created work objects). This is not a complete list, but some more info about things to think about is at:
Login with that new Guest login and make sure everything works before deprovisioning the old Member login.