It's not clear to me from reading Double-Faced Card Rules: do transformed creatures revert back to their original type automatically? And if so, when? End of your turn or beginning of your next turn?
1 Answer
Double-faced cards only transform when they (or another effect like Moonmist) explicitly say they transform. For example, Ulvenwald Mystics says on its front side:
At the beginning of each upkeep, if no spells were cast last turn, transform Ulvenwald Mystics.
and the back side (Ulvenwald Primordials) says:
{G}: Regenerate Ulvenwald Primordials.
At the beginning of each upkeep, if a player cast two or more spells last turn, transform Ulvenwald Primordials.
Thus, the conditions for transforming in both directions are printed on the card.
Loyal Cathar, on the other hand, has a transformation-causing ability on its front side but no transformation-related abilities on its back, so once it transforms, it stays that way until it leaves the battlefield.
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1Since this subsumes just about everything I was going to say (and says it better), I'll add one tiny note from my answer: this is implicit (but could be much better spelled out) in the informal article pointed to: "not all double-faced cards transform in both directions." Jun 12, 2014 at 21:49
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3Your answer covers when they transform, but the OP asked when they changed face. There's is one case of the latter that transforming doesn't cover. They change to their default face when they change zone (become a new object). You hint at this at the end, but I thought I'd mention it explicitly.– ikegamiJun 13, 2014 at 11:49