First, sorry to bump, but thats it....trials.
Now really, since this is a perm, making its skill cost 0 is too OP. Maybe make it cost 3 unupped and 2 upped. And what would really make mutation a perm is making the upped skill improved mutation. That way, unupped would be something like random mutation, kinda risky, and would make upped risky, too. What I don't like is that I don't know if I will be brave enough to use this card. My point here is, its like risking to kill one of your creature. Probably, I will use this card mostly in ranbows, instead Fallen Druid. And if you make upped skill as I said above, it will be better cause you will be able to choose from risking one of your creature to die, or risking one of your opponent's creatures gets changed into a cool creature (xD).
Something else. If you plan adding some skill cost, then you should lower the cost to play this card. If the cost stays the same, and you add skill cost, then its lot better to play some mutations, or even Fallen Elf / Druid
My suggestions here are:
Adding some skill cost;If you do that, then lower the cost to play this card to 5 unupped 4 upped (or if that is too low for you, make it at least 6 unupped and 5);Change upped skill to Improved Mutation.