Update: If you need to perform the User > Shared mailbox conversion to preserve access to mailbox of employee that has left the company, we recommend that you follow the Remove a former employee from Office 365 article first, to properly disable access to the mailbox. Then - to preserve the mailbox, perform the conversion to Shared mailbox and reclaim the license.
Here’s a scenario that might be familiar to you: Sometimes, in support-focused organizations, the email account for external communications with customers is often managed by a single employee. When that employee transitions out of that role, mailbox management responsibility can be shared by multiple employees until a replacement is in place. To provide access to the mailbox for multiple employees, admins typically convert the mailbox to a shared mailbox. Previously, multiple PowerShell commands were required to convert and reconfigure a mailbox. As a result, admins have been asking for a simpler way to convert mailboxes.
Admins can now convert a cloud-based user mailbox to a shared mailbox with a single click in the Exchange Admin Center (EAC). No more needing to use PowerShell. Similarly, a cloud-based shared mailbox can be converted to user mailbox with a single click, as well. This feature applies to cloud-based mailboxes only. There are no plans for on-premises support at this time.
Currently we support converting only between user mailboxes and shared mailboxes. Mailboxes placed on Hold, and personal archive mailboxes are also supported. There are no plans to support other types of mailboxes at this time.
Convert a User Mailbox to Shared Mailbox
As you can see from Figure 1 below, you simply navigate to the list of user mailboxes in EAC. Select the mailbox you want to change and click Convert. You will be notified when the conversion process has completed.
Figure 1 Converting user mailbox to shared mailbox
Convert a Shared Mailbox to User Mailbox
Similarly, as shown in Figure 2 below, you can navigate to shared mailboxes in EAC, select the mailbox you want to convert and click Convert. You will be notified when the conversion process has completed.
Figure 2 Converting shared mailbox to user mailbox
Be sure to add a license for the converted user mailbox and assign it a temporary password before using it. A license is required if a shared mailbox exceeds its quota. This applies to any shared mailboxes that have been converted from user mailboxes. To manage mailbox licenses, see Assign or unassign licenses for Office 365 business.