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.
Shared mailboxes in the Business and Exchange Online Plan 1 and Plan 2 have a quota of 50 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.
Both on-prem Exchange and Office 365 Mailboxes can be converted to shared either using PowerShell or using the Exchange Admin Center.
Using the Admin Center
To convert mailboxes using the web interface, open the Office 365 Admin center in your browser. Select Users then select the mailbox you need to convert.
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 again, then click Edit to the right of Product licenses. Slide the license indicator to Off then click Save.
If you are comfortable using Powershell, you can use the Set-Mailbox cmdlet to convert a mailbox to a shared mailbox.
Set-Mailbox firstname.lastname@example.org -Type shared
If desired, you can change the quota before setting the mailbox to shared.
Set-Mailbox email@example.com -Type shared -ProhibitSendReceiveQuota 10GB -ProhibitSendQuota 9.5GB -IssueWarningQuota 9GB 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
To create a list of all users and mailbox types to a csv file, use this cmdlet. Name returns the display name; you can use alias instead (or add alias as another field).
Get-Mailbox | Select-Object name,RecipientTypeDetails| Sort-Object name| export-csv D:\Documents\output.csv
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)