Shared Mailbox (convert to)

Set-Mailbox testmbx@crouse.org -Type share

Shared Mailbox (create) 

New-mailbox sharedTestmbx -shared -userprincipalname testmbx@crouse.org

   Creating a new session for implicit remoting of "New-Mailbox" command...

   Name                      Alias                ServerName       ProhibitSendQuota

   ----                      -----                ----------       -----------------

   sharedTestmbx             testmbx              ExchangeSvr      unlimited