I decided I want to change from the 1 month subscription to the 3 month subscription, can I just pay the difference?

No, this as stated in the product description, you cannot change your subscription length once purchased. You cannot change your mind and switch from a 2 week to a 1 month or a 1 month to a 3 month, etc. If you wish to extend after purchasing a subscription, you will need to purchase a new subscription at the original price.

Allowing users to change subscriptions at will would defeat the purpose of allowing discounted rates for longer subscriptions.