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.
As of October 2014, the limits on shared mailboxes in the Small Business and Enterprise plans was raised to 50 GB. Shared mailboxes in Exchange Online Plan 1 and Plan 2 have a quota of 10 GB.
A license is not required for a shared mailbox provided you do not enable In-place Archive. If you are converting a user mailbox with an In-place archive to a shared mailbox, you'll need export the archive to a pst before converting the mailbox.
Office 365 Mailboxes can be converted to shared either using PowerShell or using the Exchange Admin Center.
Using the Admin Center
To convert mailboxes in the Admin Center, open the Exchange Admin Center in your browser. Select recipients then select the mailbox you need to convert. Click Convert on the right side of the screen, under Convert to Shared Mailbox header.
It will take a few minutes to convert the mailbox to shared. Once finished, remove the license from the mailbox in the Office 365 Admin center. Select the account, then click Edit next to the Assign license information on the right.
Deselect the licenses then click Save.
Set-Mailbox email@example.com -Type shared
If you are creating a shared mailbox in an Exchange Online Plan 1 or Plan 2, set the quota to 10GB before setting the mailbox to shared.
Set-Mailbox firstname.lastname@example.org -Type shared -ProhibitSendReceiveQuota 10GB -ProhibitSendQuota 9.5GB -IssueWarningQuota 9GB Set-Mailbox email@example.com -Type shared
The Type parameter supports Regular, Room, Equipment, and Shared as values.
Verify the mailbox is now a shared mailbox:
Get-Mailbox -Identity firstname.lastname@example.org | 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)