The script is available for download from https://gallery.technet.microsoft.com//How-to-convert-mailbox-to-c92c9153. You can also use Microsoft Script Browser for Windows PowerShell ISE to download the sample with one button click from within your scripting environment.
This script will convert an existed mailbox to a new mail user.
This script is to help Exchange Administrators to convert mailboxes to mail users via PowerShell.
You can find more All-In-One Script Framework script samples at https://aka.ms/onescriptingallery