No Wizards of the Coast feat or low-LA template does what you want
There's no feat that does what you want: allowing a monstrous humanoid to change its type while keeping the features and traits of its original race. No feat simply says The creature's type beomces aberration or anything like that. Likewise, the LA +2 template insectile (Savage Species 121-2) and the LA +3 template spellwarped (Monster Manual III 162-3)—the two Wizards of the Coast templates that transform a monstrous humanoid into an aberration—, because of their LAs, are, respectively, merely interesting and inappropriate.
This means either using something else—like a spell—to achieve what you want or going beyond material published by Wizards of the Coast. Or, if being a specific aberration is sufficient, that can be done officially, but it needs DM approval of the creature's background. (If none of that works for you, you can stop reading.)
Gaining the type using a spell
The 1st-level Sor/Wiz spell aberrate [trans] (Book of Vile Darkness 84) for 10 min./level, in addition to other effects, changes a living creature's type to aberration. Usually a difficult spell for a PC to cast because of the spell's fiend component, a potion of aberrate (1st-level spell at caster level 1) (50 gp; 0.1 lbs.) remains available in all but the tiniest settlement and a wand of aberrate (1st-level spell at caster level 1) (750 gp; 0 lbs.) in small towns and bigger settlements. Neither item requires the user satisfy the aberrate spell's fiend component.
(Further, although it must meet the DM's approval, according to the DMG's guidelines (285), a custom wondrous item that provides continuously the effects of the spell aberrate at caster level 1 has a base price of 3,000 gp. Make of that what you will.)
(O, also a DM may allow funding an at-least-Wiz9's research into making the spell aberrate permanent via the 5th-level Sor/Wiz spell permanency [univ] (PH 259-60). This'll cost 5,000 gp for the research and, if successful, a Wiz9 should be able to cast the spell aberrate from a wand followed by the spell permanency at level 9 for 450 gp plus another 2,500 gp for suspected 500 XP involved with such a permanency. A DM may rule such research fails because, for example, the 2nd-level Drd spell barkskin [trans] (PH 203) isn't on the regular permanency list, but it's worth a shot, and, if the DM does agree, the wizard who was funded may allow sharing the formula with others—like the higher-level wizards that'll need to recast such an off-list permanency spell so it's level-appropriate after it's dispelled.)
Gaining the type using a semi-official but non-Wizards of the Coast template
The well-reviewed Life-shaping Handbook (2008) is hosted by Athas.org, the site having received permission from Wizards of the Coast for fans to expand the AD&D 2e Dark Sun campaign setting into D&D 3e and 3.5e, therefore making its products—if you turn your head and squint—sort of official. And, while Athas.org's products aren't without problems, rarely are other publishers' products–including Wizards of the Coast's own–without problems, either.
Anyway. The template swamp-touched creature (Life-shaper's Handbook 107-11) has its LA dependent upon what special qualities the template grants, but the only other change made to the creature by the template itself (rather than its special qualities) is that the creature's type becomes aberration. For a creature with swamp-touched special qualities totaling from LA less than 0 to LA +2/3, the template's LA is no higher than +0. (Like almost every fraction that doesn't say it isn't, it's rounded down.)
Because it's not without problems, the Handbook says the template "can be added to any living corporeal animal, giant, humanoid or humanoid creature" (109), which is nonsense, and almost certainly that second humanoid should be, instead, monstrous humanoid (as it's listed in the Min/Max Boards' laudable "Uncle Kitty's Guide to Template Based Shenanigans"). Further, the template can be gamed. For example, it's possible for a creature possessing a LA +0 version of the template swamp-touched creature to appear a perfectly normal member of its race (lacking the extraordinary ability cosmetic mutation that adds +0 to the template's LA) yet to possess a scaling recharge-rate-measured-in-rounds breath weapon. That is, if seeking DM approval for the template for an already optimized PC, this player recommends low-balling the special qualities, not picking for example, the special qualities breath weapon, cold susceptibility, DR 5/—, fast healing 5, lethargy, and pheromone repulsion for a net LA +0.
As an aside, if the DM balks at the possibility of your PC drawing its template from this source, you can point out that you're playing a thri-kreen, one of the races that was introduced to the game's canon for PC use by the Dark Sun campaign setting. So there's that, I guess.
It's official, but it's complicated, slightly shady, and probably not what you want: the feat Momentary Alteration
If it's sufficient to, instead, actually become—type and all—an aberration 1/day for 1 min. using an effect like the 2nd-level Sor/Wiz spell alter self [trans] (PH 197), the spelltouched feat Momentary Alteration (Unearthed Arcana 94) grants such an ability. The feat seems far away from what you want, but it may still be enough.
Taking the feat at level 1 requires the DM approving a convoluted character background. It goes something like this: The creature stole from its master that potion of aberrate (see above) and a minor ring of spell storing (DMG 233) (18,000 gp; 0 lbs.). Then the creature drank the potion and used the ring-stored alter self spell to assume the desired up-to-3-Hit-Dice form.1 Then the master found the creature, took back the ring, and every day for a month made the creature scrub the otyugh pit to pay for the potion.
Thereafter, the creature meets the prerequisites of and can take the feat Momentary Alteration so that it can assume the once-assumed form. For example, an anguillian (Stormwrack 136-7) or, perhaps, a choker (Monster Manual 34-5), therefore setting up the creature to take later the feat Assume Supernatural Ability (Savage Species 30-1) et al. if the DM's still using the choker that possesses the supernatural ability quickness.2,3
While this may seem shady—and nobody likes a character's mechanics hinging on the DM's approval of a character's background—to this player it appears no shadier than liberally applying multiple and increasingly bizarre and obscure templates so that one can play the character one envisions.
1 The ring "contains up to three levels of spells that the wearer can cast," so the feat's prerequisite is technically met. Also, the form is likely limited to a maximum of 3 Hit Dice because of the ring's minimum and because greed, here, is bad anyway.
2 The anguillian is an awesome eel man, so when the creature assumes the form of an anguillian and onlookers say, "What the hell, man?" you can correct them, saying, "No, what the eel-man!" I'm sorry.
3 The Monster Manual (2012) removes the choker's supernatural ability quickness. I'm not sorry.