If you have already an unlimited pack you can upgrade to a pack offering more slaves at anytime. The system will automatically compute the remaining time price and deduct it from the new subscription price.
To do this, first you need to login, then visit the menu "MyAccount -> My Plans", select your subscription and click on the "Upgrade" button. Then you can choose your new pack, you will see the cost for it, how much you earn on the current plan and finally the amount that will be charged for your new plan. If you agree, you will be redirected to the Order Confirmation where you have to select your preferred payment method and proceed with the check out.
Please note that you can upgrade to a new pack only keeping the same subscription period (1 month, 3 months, 1 year), if you want to change it and buy a pack for another subscription period, you will have to cancel the existing plan and subscribe to a new one.