Stealing a Soul Seizer
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 ...