Rule 710.4 · Flip Cards
Flipping a permanent is a one-way process. Once a permanent is flipped, it’s impossible for it to become unflipped. However, if a flipped permanent leaves the battlefield, it retains no memory of its status. See rule 110.5.
Ask the judge Pro
The life counter has an AI judge that answers your play citing the exact rules. It is part of Gatewatch Pro; the rules, Oracle text and rulings stay free.