Changeling from Eberron Campaign Setting is ideal here: you get minor change shape at will, which is very nice with warshaper class features, and its changes are purely cosmetic and therefore your alternate forms have all the features of your regular form. Changelings are an LA +0 playable race, though obviously you can’t be a changeling if you are also a thri-kreen.
You can also get the same at-will minor change shape as an egoist psion, instead of your 1st-level bonus feat. That option is presented in this Mind’s Eye article. I’m currently playing a psion warshaper doing exactly this. The −6 Intelligence penalty does mean you will probably get nothing out of the psion’s powers, sadly, since you need 11 Intelligence to use them, which is basically not going to happen with a −6 penalty. Still, for one level, to get at-will access to warshaper features, it may still be worth it.
Now, there is a discrepancy between the two we need to address: changelings have the shapechanger subtype, which is defined as follows:
A shapechanger has the supernatural ability to assume one or more alternate forms. Many magical effects allow some kind of shape shifting, and not every creature that can change shape has the shapechanger subtype.
The shapechanger subtype is one of the listed ways to qualify for warshaper, which specifies
Special: Must be able to change shape in one of the following five ways:
- Change shape supernatural ability (aranea, hound archon, barghest, doppelganger, rakshasa, slaad).
- Shapechanger subtype (lycanthropes, phasm).
- Polymorph as a spell-like ability (astral deva, planetar, solar, couatl, marilith, bronze dragon, gold dragon, silver dragon, efreeti, leonal guardinal, night hag, ogre mage, pixie).
- Able to cast the polymorph spell.
- Wild shape or similar class feature (bear warrior, druid).
The alternate form ability (possesed by quasits, vampires, and others) is insufficient to become a warshaper.
As you can see, this is a rather specific list—and minor change shape isn’t on it, and is not the same as the change shape ability of a doppelganger or others. So while changeling definitely qualifies, due to the shapechanger subtype, it’s not clear if an egoist—who has minor change shape but nothing says is a shapechanger—does.
Then again, warshaper was published in Complete Warrior, long before Eberron Campaign Setting and changelings ever existed, and thus before the Mind’s Eye article gave minor change shape to egoists. Therefore, we can’t read too much into the omission of minor change shape from the list—it didn’t exist to be listed in the first place. We can’t read that as an intentional choice to exclude it. And minor change shape is much more like change shape than it is like alternate form. A creature with one or more alternate forms always turns into the same form—you can learn to recognize a vampire in wolf form, because they always turn into the same wolf. A druid can wild shape into any wolf they please. A changeling—or egoist—can take on the form of any individual with the same body type. For my money, that’s a good reason to consider minor change shape as sufficient to qualify for warshaper.
We can also consider this from another angle—the egoist option for minor change shape was published in a Mind’s Eye article. The Mind’s Eye articles are done to a pretty significant degree of polish, among Wizards’ web articles, but it’s still less than that of a full book. That article credits Dark Psion and Mark A. Jindra—just two people. Eberron Campaign Setting had three game designers, three game developers, three editors, and a manager for each of those teams, as well as a “Director of RPG R&D.” Clearly, Eberron Campaign Setting got a lot more care—it’s also a vastly bigger undertaking, but you see my point, there are a lot more eyes and a lot more time spent dotting i’s and crossing t’s. Which is to say, I think egoists with minor change shape should have the shapechanger subtype. Not every creature who can change forms gets that subtype, but changelings do—and the only reason they have it is because of their at-will minor change shape. Egoists with this option have the exact same ability, and so should also be shapechangers. And if an egoist has the shapechanger subtype, that absolutely qualifies for warshaper.
So either way, I think egoist should be an option for warshaper.
Aside: Insectile Creature
Depending on why exactly you’re interested in thri-kreen, you may want to consider Savage Species’s insectile creature template as a possible alternative.
Addendum: Pathfinder’s minor metamorphosis
Disclaimer up-front: I have worked freelance for Dreamscarred Press, whose work I am going to recommend, in the past. I did not work on the particular product I’m recommending, and have no financial interest in its success (also, all of it is OGL and available on d20pfsrd.com, so you don’t have to buy anything anyway).
I am playing an egoist warshaper in a campaign, and while my character was ruled to have the shapechanger subtype, and minor change shape was ruled sufficient for entry to warshaper anyway, it’s worth noting that one of the key things that my character relies on is also an excellent answer to this question—if you can use some third-party Pathfinder material.
Dreamscarred Press (which did some 3.5e material before switching to Pathfinder, and which has produced almost all of the psionic content for Pathfinder 1e) adds a 1st-level power to the egoist discipline list called minor metamorphosis. Unlike 3.5e metamorphosis and greater metamorphosis, the Pathfinder versions (as well as the new minor metamorphosis) don’t have the new form’s statistics replace your own, you just get some cosmetic changes as well has a few specific new forms of special attacks or abilities.
That makes minor metamorphosis a valid answer to this question, because you keep all of your abilities while using it. It’s also a strong ability for a warshaper to have. There is some question as to whether it’s a valid option for qualifying for warshaper, but metamorphosis is very close to simply being the psionic version of polymorph, listed in the options for warshaper. Thus, it is, I think, even more clearly a valid option than minor change shape.