Hi guys,
I have recently went through this process and would like to share info from support.
Thank you for reaching out to Mailgun Support. Currently there isn’t a downgrade path available in the UI, or here in support from the basic plan to the flex plan. You will have to close this account, create a new one on the foundation or scale trial, and at that point you will be able to downgrade to the flex plan. We also recommend going over the specs of the new free plan that is present on the Mailgun pricing page as that is going to be replacing the flex plan, and the flex plan will no longer be available soon