First of all, you get the same code and functions in both license types.
According to Envato rules, you need to have an extended version to charge end clients.
So with an extended version, you are legally covered.
Yes you can, and you will pay only the difference. You will need to follow this guide.
You can ( the system has all the functions ).
But it is not legal. So I will suggest to try the system and even charge some users with regular licensee, and then if business is good, ask for Extended license.