Many years ago I established an outlook email account using email@example.com but never used it.
When I initial purchased my Office 365 Home subscription, forgetting about the previous firstname.lastname@example.org I used a username that was email@example.com. I shared that subscription with two other family members who used firstname.lastname@example.org which allowed them to use their attached Onedrive space after a single login using email@example.com. They also used firstname.lastname@example.org for their daily email.
I contiued to use my gmail (email@example.com) and never actually used my Office 365 account for email.
Yesterday I decided to abandon firstname.lastname@example.org and attempted to see if I could change my email@example.com Office 365 username or add an alias to regain use of firstname.lastname@example.org. In talking with Microsoft help I was told I could change the account username and transfer my subscription but it would result in having to reinvite those other two family members and that all of us would lose the signifcant amount of data stored on our onedrive linked accounts. That obviously is a no-go.
I was also told that in order to use email@example.com as an alias I would have to delete that account and wait 60 days for it to "clear" so it could be used. Even tho that would allow use of firstname.lastname@example.org for email it wouldn't change the login for my master office 365 account and its associated onedrive.