It is your body you change not your DNA, why? Because of two things in the spell description. You retain human consciousness, which you would not if your DNA changed because that would change your brain as well. Two, there's a restriction on what you can change into that is body +/-2, which to me clearly suggests that it is a bodily change not a genetic change, since such a restriction would be meaningless if your DNA changed, simply put if you have the dna of a gnat you're a gnat, if you have the dna of an elephant you're an elephant.
Therefore I still say you need magic that breaks the rules, hence the need for powerful artifacts, mentor spirits or some such. But it's your game of course, so your rules.