Earlier in the game, triggered Sigil of Alacrity and quest on the same turn, getting ilgynoth to cost 4. I played kurtrus. Later, celestial alignment was played, making ilgynoth cost 1 in my hand. I played glide, redrawing Ilgynoth, but it cost 4 instead of 0.
Cards that go back a step (like battlefield to hand or hand to deck) lose all changes to them, including buffs and altered casting costs.