How about in Rainbow, where 4-cost critters are common? I assume it changes into one at random.
Thematic-wise, why don't this be in
![Entropy :entropy](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/entropy18x18.png)
? Weak creature that would evolve after an attack sounds more like a growing mutant.
Also, the upped one is doing too much. Instant effect, buffs carried over, cost reduction. At least make them cost the same.