If this were just a flat out "steal target creature from opponent", I would most definitely say no way, but the mutation aspect definitely adds quite a bit of spin to the situation. Now it's not necessarily any better than assured creature control for the unupgraded, since 'mutation' has a chance of killing off the target anyways. Unlike twin universe, you aren't going to get a copy of the target creature; you may very well end up with a creature much worse than the creature you targeted, considering that in general you will target high-cost creatures with high stats. I think that the unupgraded's cost should maybe drop by 1, because of the chance of the death of the creature. The upgraded, on the other hand, I think is fairly balanced because you will definitely get an improved mutant, leaving no chance of the caster of this spell to not get a new creature out of the deal. I personally like the idea of creature manipulation; after all, darkness has the steal card, which outright steals a permanent from the opponent, guaranteeing you not a "mutation" of the permanent, but the exact permanent you steal. Why should rules governing creatures be so different so as to not allow for even a mutated, twisted quasi-"stealing" of a creature for a much higher cost?