Converting Office 365 User Mailbox to Shared Mailbox

First connect to Office 365

Connect to O365

$LiveCred = Get-Credential

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection

Import-PSSession $Session

Then

Set-Mailbox -Identity mailbox -Type:Shared

Set-Mailbox mailbox -ProhibitSendReceiveQuota 10GB -ProhibitSendQuota 9.75GB -IssueWarningQuota 9.5GB

If your mailbox keeps getting converted back to user mailboxes you may need to try this – http://support.microsoft.com/kb/2710029