Unless WotC changed that, a superinfection with a new tadpole would either be just a regular brain eater who wouldn't need to adapt (but maybe would refuse "cannibalising" the remains of the Absolute tadpoles), or - given the outcome of the game - a new Absolute tadpole in the sense that it would act like the ones we see in the game. Unfortunately, I don't know many of the endings and also don't want to given the potential chance of another playthrough.
The normal ceremorph, according to "The Illithiad" (2e), states that Gnolls, as well as Gith, humans, M-sized goblinoids, grimlocks and elves would be turned into mind flayers, while unacceptable hosts (Halflings, all dwarves, gnomes, centauroids, giant-kin, Kuo-Toa,...) turn into a variety of monsters or simply die during the metamorphosis. This also fits to the Absolute tadpoles being different.
