It’s not often that you’ll need to convert a mailbox to a mail-user, but when you do, you’ll soon realize the steps go like this:

1. Mail-Disable the user (delete the mailbox)
2. Mail-Enable the user

So what’s the problem?  The problem is twofold:

• First, you’ll want to automate this, and there is no “convert” button or command.  You’ll need to use PowerShell if converting multiple users.
• Second, and perhaps more importantly, all the Exchange attributes are nullified when you delete the mailbox.  This includes CustomAttribute1-15

As we can see, you are not able to pass mailboxes to the Enable-MailUser (as you are able to do in reverse):

I’ve written a script to solve these problems.  Before you run with it, you do need to make one decision:

What do you want the mail-user’s external email address to be?

The below script takes the user’s mailbox alias and then appends @domain.com.  You may wish to modify this with whatever their new external address has become.

You’ll also notice I’m using a static domain controller for all configurations.  I have found in my testing, that if you do not pick the same DC for all operations, the script could out-run replication.

$DomainController = (Get-ADServerSettings).DefaultConfigurationDomainController.domain

$MailboxList= Get-Mailbox

foreach ($Mailbox in $MailboxList) {
    Disable-Mailbox -Id $mailbox.Identity -Confirm:$False -DomainController $DomainController
    Enable-MailUser -Id $mailbox.Identity -ExternalEmailAddress ($mailbox.alias +"@domain.com") -DomainController $DomainController
    Set-MailUser -Id $mailbox.Identity `
     -DomainController $DomainController `
     -CustomAttribute1 $Mailbox.CustomAttribute1 `
     –CustomAttribute2 $Mailbox.CustomAttribute2 `
     –CustomAttribute3 $Mailbox.CustomAttribute3 `
     –CustomAttribute4 $Mailbox.CustomAttribute4 `
     –CustomAttribute5 $Mailbox.CustomAttribute5 `
     –CustomAttribute6 $Mailbox.CustomAttribute6 `
     –CustomAttribute7 $Mailbox.CustomAttribute7 `
     –CustomAttribute8 $Mailbox.CustomAttribute8 `
     –CustomAttribute9 $Mailbox.CustomAttribute9 `
     –CustomAttribute10 $Mailbox.CustomAttribute10 `
     –CustomAttribute11 $Mailbox.CustomAttribute11 `
     –CustomAttribute12 $Mailbox.CustomAttribute12 `
     –CustomAttribute13 $Mailbox.CustomAttribute13 `
     –CustomAttribute14 $Mailbox.CustomAttribute14 `
     –CustomAttribute15 $Mailbox.CustomAttribute15
     }

(add more attributes if necessary, but remember that since you aren’t deleting the Active Directory object itself, most attributes remain…)