the one canon example we have, she tried pretty hard to get herself killed during the spell (though that could have been about the convenience of making sure the body she left didn't get uppity).
My comment that it might have to be run as a death curse is that if someone were to try it as evocation (without any sponsored magic to back it up), there'd be no other way to gather enough shifts of power to pull it off, and I think that's appropriate.
You're talking about hot-swapping someone's mind and soul, without any copy-errors here. Not something you do easily on the fast and loose, period. I think you should have to force a take-out in order to pull it off.
Sure, sure, that's a lot easier if you've already beaten your opponent down (mentally, one would assume, since you're going from that to major mental mojo, and you'd want those consequences to tag), but then it's not being accomplished as a single exchange, either, strictly speaking. It's being accomplished as the culmination of a combat, and I could be convinced to simply allow it as the take-out in a mental combat (probably involving some sort of FP bribery on the part of the person doing it, in order to get the victim to accept that take-out).
Meaning that it could be done without a death curse, but only with a solid foundation of consequences and more than a couple FP.
Which is why I'd recommend doing it with thaumaturgy.