After a user quit, an administrator wanted to convert the mailbox to a shared mailbox, to free up the license the mailbox was using. Because the user handled important communications, he didn’t want to delete the mailbox right away.
While I’d export the mailbox to a pst for backup, delete it and add the user’s address(es) to the mailbox of the person who took over the job, it is possible to convert a mailbox to a shared mailbox.
Because shared mailboxes are limited to 5 GB on Office 365, you first need to reduce the quota to 5GB. If the mailbox is larger, you’ll need to move content to a pst file or delete it. If the mailbox has an online archive, export it to a pst file.
Set-Mailbox email@example.com -ProhibitSendReceiveQuota 5GB -ProhibitSendQuota 4.75GB -IssueWarningQuota 4.5GB Set-Mailbox firstname.lastname@example.org -Type shared
The Type parameter supports Regular, Room, Equipment, and Shared as values.
Verify the mailbox is now a shared mailbox:
Get-Mailbox -Identity email@example.com | Format-List RecipientTypeDetails
After converting the mailbox, give other users Full Access and Send As permission as needed. You can do this from the Exchange Control Panel or using PowerShell.
To use PowerShell to give a user or group FullAccess rights:
Add-MailboxPermission username -User newuser -AccessRights FullAccess
To use PowerShell to give a user or group SendAs rights:
Add-RecipientPermission username -Trustee newuser -AccessRights SendAs
Now you can remove the license from the account in Users and Groups. Two dialogs will warn you that removing the license will delete the mailbox and the contents, however, because you converted the mailbox to a shared mailbox before removing the license, the mailbox won't be deleted. Note: while the mailbox contents are safe, Lync contacts may be lost.
The second dialog reads:
Are you sure you want to remove the Exchange license?
The user’s mailbox and all messages in it are deleted. We recommend that you only delete licenses from users who no longer need email.
All email aliases associated with the user account are deleted. If you need someone in your company to receive email at those aliases, assign them to another user.
Their Lync Contacts list may be deleted. To restore their Lync Contacts, assign the user an Exchange license within 30 days. If you want to remove their Exchange license but keep their Lync Contacts, contact Support before you remove the Exchange license.
More information is available at Convert a Mailbox (TechNet)