The documentation describes the attributes supported for Office 365 Mailbox objects, which includes attributes such as the IsEqipment, IsRoom, and isShared. How to properly provision such objects?
Room, Equipement and Shared Mailboxes must be created as a standard mailbox object with the attributes IsRoom, IsEquipment, or IsShared attributes set to 'True'. When Office 365 gets the request, an associated user object will then be created and transformed into the desired mailbox type.
If updating a regular mailbox to a shared you will need to set the 'IsRegular' from 'True' to 'False' and then set the 'IsShared' to 'True'.