from https://tuta.com/support/general#subscribe

If I upgrade, can I downgrade to free again?

Yes, you can downgrade back to free anytime. Before this, you need to disable all extra bookings. You can keep your main Tuta email address as a free account.

just to be sure, does this mean that i can keep using me@tuta.com even if i downgrade to free? What if i already had me@tutanota.com before i upgraded?