Will i be charged double if i upgrade for a plan when i have already paid for a different one?

Appointo is a 3rd party partner of Shopify. The billing is handled by Shopify and all your payments for Appointo will show up in your next Shopify invoice. Shopify does a great job in handling the payments and makes sure that you are not double charged irrespective of how many times you downgrade or upgrade the app. You can freely choose to do so and you would only be charged based on the plan you are in.