Your regular billing cycle date will remain the same if you change your subscription. Any add ons will be charged immediately and activate upon completion of sign up. If you add a service midway through your monthly subscription, the cost of the service will be prorated for the remainder of your monthly service. On your renewal date, you will be charged the full price for your new service.
If you choose to remove an extra service that you currently subscribe to, the change will take place on your renewal date, meaning you will be able to watch your previous subscription until that time.