here is the response from their support team:
Hello,
Thanks for reaching out! Billing frequency cannot be changed once your subscription is active, so in order to change to annual billing, you will first need to downgrade your app to Free and then re-upgrade to the plan of your choice. You can change your app’s plan by going to your app’s editor, clicking on Settings, then the App Plan tab; you can then use the dropdowns to select your plan and billing frequency. Additionally, please do keep in mind that downgrading to Free cancels your plan immediately and any unused time is not credited.
When you downgrade to the Free plan, your paid features are not deleted or altered, but rather disabled for the amount of time that the downgrade lasts. When you reupgrade, these features are intact and enabled. This includes custom domains, files, app data, and perpetual license plugins, etc. If you have monthly subscriptions to plugins, they will need to be resubscribed to upon reupgrade.
Feel free to reach out if you have any more questions and we’d be happy to help!
Sincerely,
This basically reinforces the lack of thought put into their licensing model!! I feel sorry that their support team is put into the situation of having to deliver a message that soooo not customer friendly.