An error occurred while saving the commentYtsejamer1 commented
One way after converting the O365 User Mailbox to a Shared Mailbox in your O365 portal is to revisit the AD account and go into the attributes for the mailbox user.
Now update the following attributes with these values:
That will switch it on your on-prem directory, which will then sync out to O365. Then your management utilities will show the correct mailbox type.
As always, be very careful when hacking around with AD attributes.