Oui oui, on peut retourner une créature avec la mue à tout moment où l'on peut jouer un éphémère (il suffit d'avoir la priorité), et cette action ne passe pas par la pile. Tu peux faire cela à n'importe quelle étape de la phase de combat.
502.26a Morph is a static ability that functions in any zone from which you could play the card it’s on, and the morph effect works any time the card is face down. “Morph [cost]” means “You may play this card as a 2/2 face-down creature, with no text, no name, no subtypes, no expansion symbol, and a mana cost of {0} by paying {3} rather than its mana cost.” Any time you could play an instant, you may show all players the morph cost for any face-down permanent you control, pay that cost, then turn the permanent face up. This action does not use the stack. (See rule 504, “Face-Down Spells and Permanents.”)