The licensing is a mess when it comes to upgrades.
Ask 5 people at Mitel and you'll get 7 different answers.
From experience, if you upgrade a stand alone controller then add a second controller, the license changes. Now you need to buy Enterprise Licenses.
Now you also run into problems with that the base software no longer has a limit as to how many licenses you can add, but... if you upgrade a controller, then move the licenses, those licenses are still limited.
I still haven't gotten my head around it all and it looks like Mitel hasn't either.
Ralph