Exchange Server 2013 Sent Items modification and the normal mailbox ?

People,

I've got Exchange Server 2013 that is just on the SP1 with the build number Standard Version 15.0 (Build 847.32)
When I execute the below script to modify the sent items to be copied in the mailbox, using the script:

Get-Mailbox HelpDesk | Set-Mailbox -MessageCopyForSentAsEnabled $true -MessageCopyForSendOnBehalfEnabled $true

I got this error message:

MessageCopyForSentAsEnabled can only be set on shared mailboxes

From the Powershell result of this mailbox properties, I can see:

IsShared : False

So what can I do to make that Powershell run with no issue ?

Is it possible to change the mailbox type to Shared mailbox without causing the user to lose access or any outage ?

Do I need to modify the user Registry for anyone using the Shared mailbox now ?
Note: The current mailbox "Helpdesk" is working fine (all of my IT team are assigned Full Access Permission in the Delegation tab) until I found out the strange behaviour today.

Thanks.