I can't remove their on the fly genetic adaptation as a whole, given it is the main way they utilise themselves in combat and adapt to situations in the moment, the main boon of millenia of war, at least not without major changes. Think of it more like mutation than some sort of magical shapeshifting twilight style. It's painful, there's lots of cracked bones and it goes wrong with a fairly high number of cases, leading to insanity and death. That's the main reason they don't risk reducing their optimisation unless there's a damn good reason to.
I can, however, remove their explicit ability to become anything even resembling a humanoid form by this odd race-wide memory of what a humanoid is (which I think I will actually do regardless, it doesn't really make sense). Make it from "oh here's a batlike person now they become a huge bat monster" to "here's a huge bat monster, which can become marginally less terrifying but is still a bat monster"
To be honest, they've never seen a humanoid race (maybe bar the Pookas, which are muroidea not humanoid) so they would have no reason to prefer that sort of bipedal form in the slightest. Especially not some squishy pink thing, yuck. Their entire social structure is based around who has the biggest fangs, so to speak.
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I would vastly prefer Legix's idea, that's essentially what I've been gunning for. You're not a humanoid that becomes a monster, you're literally a monster that maybe reduces those qualities for very short amounts of time very rarely. Even then, you might screw it up and become a Fomorian (mentioned in the faction article as a non-playable enemy species which are essentially cronenburg cthulu things which are insane and have no logical thought). Most logical Agarthans would never risk it unless they really had to, such as being captured (honestly they'd rather be executed for the reputation of their clan) or such like. They might do it for diplomatic reasons though, it might ease tensions to not sign a treaty with a huge wolf thing on the other side of the table.