Is this too strong?
Them: X flux + 1 card
You: X+1+Y flux + 2 cards.
So you get some good creature for 1+Y flux where Y is usually 1 or 2.
This guy is often closer to their base.
Sure they end up with your old creature but he is usually dead or useless.

So the only real draw back is that you have to have a creature in hand to give them. Is this enough?

I tried this in a Nether Plasma deck a while ago and it didn't seem too strong. Haven't tried it in a deck with weenies though.

It's devastating in situations, where the opponent has in play some creature that has somehow become much stronger than its flux cost would imply. Examples include carrion feeder, trained or melded creatures, and so on. Also stealing a Blind Assassin for 4 flux and glub bird or similar creature is a bit outrageous.

What I'm trying to say is it's strong but not too strong when used on most 1-2 domain creatures that haven't grown in any way. But what makes it problematic in my eyes, is that flux cost doesn't always reflect the strength of the creature.

Meld and Carrion Feeder don't get much play anyways. I'm all for more cards that serve as counters to training (and am ok with a counter to Blind Assasin which is one of the stronger creatures from the original set).

I'd be fine with adding a domain restriction so you can't change a 1 domain creature for a 3 domain creature. nothing stopping you from swapping out a creature that's going to die, just makes it a little less OP.

It's very power full. It actually works like (don't remenber the name now) Sylvian enchatment that enchant enemy creatures to give you control of it. Sylvian enchatment is 3 domains.

What make this card very powerfull is that it is a spell. That mians that when it is casted there is no way tod eal with it. I think it should be an enchatment so one can do something about it.

This card is very unbalanced, im waiting to see way to deal with it ...

The thing just do like that, u put a good creature, and the enemy put a junk and trade the mind of the two. I believe that it turning to be a enchantment could mix better... so u can someway deal with it. Like the charm!!

This balance change, of waiting for some creature to come, well, its not suficient, get a smart player and he keep a creature at hand to make it up.. always.

Well it looks to have a weakness against wennies decks.. anyway its situacional (in favor of the owner of the spell)

Thats my humble opinion, all the games i played against it where trash for me :p
Im waiting to test it against a wennie deck!

strange is, that it's market cost is still about 120g, which definitely does not fit to your description of great card.. =-O