The reshuffling of neurons during fruit fly metamorphosis suggests that larval memories don’t persist in adults

For a hot minute, I moderated a tiny niche subreddit called /r/AskSciFiHistorians (as in "ask historians of science fiction about the history of sci-fi"). Have I become the analogue of that subreddit here on Tildes, to be summoned on your whim? :P

Regarding @ChingShih's speculation about science-fictional species that undergo metamorphosis, my first thought was the species of Ploorans in E.E. "Doc" Smith's Lensman series. There's a plot point that various good guys occasionally detect the mental signature of a species that appears to be the next level up in the hierarchy of the bad guys (possibly the top-tier bad guys themselves), but each good guy who detects this mental signature "sees" an entirely different species, so they don't connect the dots until it's almost too late. The planet Ploor orbits around a strongly variable star, with conditions on the planet ranging from furnace to freezer. Therefore the Ploorans' physical form goes through extreme metmorphoses to cope with each set of seasonal conditions. (At one point they become interdimensional! The science in Smith's space operas wasn't always the best.)

There's a species in Larry Niven's Known Space universe known as the Pak. Humans are a devolved form of the Pak. In their original form, the Pak go through three stages of life: Child, Breeder, and Protector. The Child and Breeder forms of the Pak are basically human children and human adults. However, on the Pak homeworld, a middle-aged Pak enters a stage where they must eat a fruit from a plant called Tree-Of-Life. This fruit causes a metamorphosis in the Pak Breeder to turn it into a Protector: its skin thickens and dries, its joints swell up, its teeth fall out and are replaced by a bony ridge, etc. (Niven says the Pak Protector form was inspired by imagining if the human aging process was supposed to be beneficial, rather than detrimental.) Basically, the Protector becomes the ultimate fighting machine to protect the Children and Breeders.

And then, of course, there are the aliens in Asimov's novel 'The Gods Themselves'. The juvenile forms, known as Soft Ones, are wispy, thin, flimsy bodies that can merge with each other in triplets: the aliens have three sexes, rather than two. In this merging, three bodies literally become one, and the juveniles retain no memory of what happens during each merge. Initially, the juveniles do this for fun, and each merge is only temporary. However, this is a precursor to the final permanent merge which results in a metamorphosis into a final adult solid form, known as Hard Ones, comprised of all three juvenile bodies, but ultimately its own person. It is implied (but not confirmed) that the adult form does not remember what its precursor juveniles remember - as per the article linked above.

As regards bio-engineered humans, that's practically a trope in science-fiction. Genetically engineered humans are everywhere. Greg Bear's 'Eon'. Kim Stanley Robinson's 'Blue Mars'. The movie 'Gattaca'. Star Trek's character Khan Noonien Singh. And so on.