Proton AG (Proton Mail's parent company) lets you switch in both directions. Going from a paid version of Proton Mail to the free version is easy. But of course, you lose access to all the premium features. Going back to a paid version after downgrading to free is also possible (but you should probably make up your mind at some point). Just subscribe to the paid version again and get back to work. All your messages get carried over despite the flip-flopping.