Bdtyktl

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)

The Soul Seizer remains an enchantment, but you regain control of it (at the end of the turn). Nothing in the rules about the transform ability says it becomes a different permanent, even though it changes types.

701.27. Transform

701.27a To transform a permanent, turn it over so that its other face is up. Only permanents represented by double-faced cards can transform. (See rule 711, “Double-Faced Cards.”)

Since it now enchants one of your own creatures, it's effectively useless. However, you can't use it anymore on your opponent's creatures, so it's definitely a smart move from your opponent. Just not that smart ...

The game only checks if the target is valid when the spell is cast, and again when the spell resolves. If those checks are satisfied, then then Act of Treason creates a continuous effect that changes control of the object, no matter what it happens to be. The continuous effect still works even if the object no longer matches the criteria of that allowed the object to be targeted initially.

Enchant Thing, Equip Thing and Fortify Thing are different. If the object no longer matches the quality, the Aura/Equipment/Fortification falls off.