You can upgrade or downgrade an active FooEvents License directly from the FooEvents Account page. Upgrades are pro rated according to the original purchase date and amount already paid while downgrades are not pro rated but will affect renewals.
Here are the instructions for how to do this:
2. Click on Subscriptions in the left navigation menu
3. Click View next to the subscription that you would like to modify
4. Click the Upgrade or Downgrade link below the product
5. Select the product that you would like to switch to
6. Complete the standard checkout process
Your new FooEvents License and subscription will be active as soon as the payment process is completed and you will have to access any new plugins from the Downloads page.