CONVERTING USER MAILBOXES TO SHARED MAILBOXES IN OFFICE 365
As I described earlier, a shared mailbox migrated to Office 365 using a staged or cutover migration will be created in Exchange Online as a user mailbox. User mailboxes require licenses or they are deleted in 30 days. To avoid consuming an Office 365 license you can convert the user mailbox to a shared mailbox very easily.
In the Exchange admin center navigate to Recipients and then Mailboxes, and find the mailbox you wish to convert. After selecting the mailbox you’ll notice a Convert to Shared Mailbox option to the right of the page.
Click Convert, then click Yes to confirm. The conversion process takes just a few moments, and no data is lost in the conversion. When the conversion is complete click Close.
When you refresh the Mailboxes view you should no longer see the converted mailbox. Instead you can look in the Shared view to see it. Notice also that there is an option to convert it back to a regular mailbox if you need to.
Click the Edit icon and then Mailbox Delegation to add permissions for any users who need to access or send as the converted shared mailbox. Alternatively you can do this using the PowerShell commands demonstrated earlier in this article.
As a final step, if the user/mailbox had been assigned an Office 365 license before the conversion you can remove the license. However, do not delete the user account, as that will delete the mailbox as well.
As you can see shared mailbox creation and conversion in Office 365 is quite simple. This is an important concept to understand because of the implications for licensing. If your company is spending Office 365 licenses on shared mailboxes they are quite possibly wasting money, and you have the opportunity to save some of those costs by using the correct mailbox type instead.