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It won't be used for receiving email and will not be using Office 365 outside of the administration panel. If you choose to create a second administrator account, you do not need to assign a mailbox to it (therefore, no license is needed). If the account you want to change is the administrator account (common among SOHO users), you either need to make a new admin account and log in with it to reassign the primary email address of the first account or use the set-mailbox cmdlet. In Office 365, the administrator can log in and change individual users or use a cmdlet to change the domain used by all users. It usually takes longer to log in than it does to make the change.
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In a local Exchange server, changing it is as easy as the administrator logging into the server and changing a mailbox policy.
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