ProtonMail offers a variety of different plans.
You can upgrade or downgrade your account at any time. You will receive a prorated price based on the amount of elapsed time the chosen plan was used.
To downgrade your account you will need to ensure any custom domains and addresses are disabled to match the specs of the new plan you are downgrading to. If you fail to do so prior to downgrading your account, you will see the following error message stating “Error: Please disable all ProtonMail aliases and custom domain addresses before downgrading to the free plan.”
You can disable the custom domain and addresses within the Domains and Addresses tab within the Settings. Next to the custom domain you would like to disable you will see a trash can icon to delete the domain.
In the Addresses tab you will see a link to Disable next to the address you would like to disable.
When you disable an email address the address will no longer be able to send/receive mail.