If I upgrade my subscription will my initial plan be automatically cancelled and upgraded so that I don't pay for two subscriptions?
When you upgrade subscriptions your old subscription will be prorated into your new subscription.
For example, if you are on a $8.99 per month subscription and upgrade to the $19.99 option, you will be charged charged prorated amounts for the time spent on each option. Assuming the change occurred halfway through the billing month, you are billed an additional 5.50 USD: (- $8.99 / 2) for unused time on the initial price, and ($19.99 / 2) for the remaining time on the new price.