Yes, that is correct. The only change you would have to make on the existing sites would be to activate the new support license keys on the site you wish to use them on.
I think the least expensive option would be to upgrade your purchase of the Extension Pro Pack to the the Pro Pack 1–5 sites option.
To do this navigate to your purchase history, click the Licenses tab and then click the View Upgrade Options link for the Extension Pro Pack. There you will see the option to upgrade to the Pro Pack 1–5 sites option.
Let me know after you do, so I can adjust the license expiration. I need to do this manually because unfortunately upgrade do not extend the license date.
Hope I have answered your question!