Let’s come up with a monster.

How about this option :
I don't know what to call it.
1. 40 hp. no abilities (did poorly in school :) )

Cool! Call him "the truant"; and the upgraded one - 60 hp, he has the ability to vanish - invisible to the enemy until the first attack (like, he learned by hiding from teachers ;)) - "the slacker":D:D:D

But seriously, I like the idea of metamorphosis! Only I would do it this way - for metamorphosis, this unit must have killed at least one creature of those it intends to turn into. And that metamorphosis should take a full turn on the AVT-scale and last no more than 2 turns (otherwise it'll be too overkill)