If you already have a Drivemob account nothing will change with your subscription when you upgrade to a new Softbox 3. Your drivemob subscription will work on the new box and stay active.
When you order the box you will be sent an email asking about your order. After you reply to the email, you will be sent an Management code that you will need to activate your new box on your current Drivemob subscription.
If you don't have a Drivemob account. You will need to create one when you follow the set up instructions to your Softbox 3.