Change email alias to on.microsoft.com account as primary

Hi,

I'm performing an O365 tenant-to-tenant migration. I need to remove the domain name for the source domain and register and verify it in the destination domain. I'd like to make sure that all users using the alias domain I'm migrating to the new tenant say (@testdomain.com) to be set to use (@testdomain.onmicrosoft.com) as their primary address in the source tenant so that I can move the alias.

I'd like a script that lists all accounts using @testdomain.com as primary and change it to @testdomain.onmicrosoft.com 

so far I've come across two scripts that do both parts but I'd like assistance with making it one complete script. The two scripts I've come across are below. If there is a better way of doing this please let me know:

Get-MsolUser -DomainName Fabrikam.com -All

Set-MsolUserPrincipalName -UserPrincipalName user@testdomain.com -NewUserPrincipalName user@testdomain.onmicrosoft.com