I feel like the problem with Kheldians isn't Changeling, it's the modularity of the class making decisions you make for one form worthless for the other forms.
Consider, if you will, the Druid class from WoW, which is similarly modular. The difference is that in WoW, you can build a single form of a Druid to do everything it needs to do for viability in a primary role: you can build a Bear Druid to tank, and it will tank as well as any other tank, and you can largely ignore the other forms except for utility.
You don't really get to do this with Kheldians. Sure, I've seen Human-only builds that function "fine", but we all know that tri-form is optimal, and the optimal way to play tri-form is via the "Changeling" build. Getting upset at people for doing what is optimal is both unreasonable and unfair.
The better solution is to make the "forms" extensions of the baseline Human form and NOT have all of the benefits of Human form "shut off" in the other forms. "Changeling" exists SPECIFICALLY to allow Human form to use its abilities alongside the other two forms -- all at once. The problem arises from the fact that this is NOT how the class was designed.
There needs to be a reason to change forms and STAY in that form for at least 15 to 30 seconds so that the form shift is MEANINGFUL. I don't have any specific ideas, but I DO feel like Human-form toggles should stay active in the other forms so that those forms can feel like they are fully-fledged characters in their own rights, but that are lacking critical elements that making staying in any one form permanently sub-optimal (but still viable!).
Honestly, the easiest way to stop the whole "Changeling" problem is likely to give Human form a short cooldown like the other two forms; in other words, you can't shift back to Human for 2 to 4 seconds after transforming.