I was in control of a Soul Seizer that I cast. An Act of Treason, which states "gain control of target creature until end of turn", was used to take my Soul Seizer from me. My opponent then attacked and damaged me with it, chose to transform it, and then took one of my other creatures, and the Seizer became an enchantment. What happens here? Does it return to me immediately because it's no longer a creature and the effect granting control targeted a creature. Does it remain where it's at (in my opponents control) permanently because it became something else in his possession. Or does it return to me when the effect ends even though it's no longer a creature.
I believe it comes back because the effect ends, but my opponent felt he should keep it because it became something else while under his possession. I couldn't find an answer and I would be curious to know the specific rulings. (I let him keep it because it didn't matter and I defeated him anyway)