[Membership 2 Pro] Explaining Upgrading & Downgrading Memberships

How does upgrading and downgrading subscriptions work? In this use-case a user should only have 1 subscription at a time.

If a user has "Pro" and downgrades to "Simple" - does the content restriction effects happen immediately? Does the user get billed immediately or only when the "Pro" plan has expired?

What about the reverse when a user upgrades from "Simple" to "Pro"?