MDT: Change the Windows product key quickly with Unattend.xml

Here's a quick and easy way to change the product key for an OS in MDT:

1. Use Notepad++ or some other advanced text editor to open the following:
   "<MDTSharePath>\control\<TaskSequenceId>\unattend.xml"
2. Locate your product key under the "<ProductKey></ProductKey>" entry. 
3. Replace the key under that entry with your updated product key and save the file.