Tetsuwan Atom (1980) - episode 52 - 22 Dec, 1981

In a prologue, Astro Boy's creator Osamu Tezuka reveals a little secret about the robot boy: That his legs are actually those of a robot girl. The backstory behind this fact, the plot of this episode, is that Astro Boy is tasked to retrieve one of his own blueprint templates from a military fortress of the Grota Republic. He meets and bonds with Nuka (Nikki in the American dub), a sentient robot girl built from his very template, which unfortunately incorporates a neutron bomb, and in order to stop the bomb, the two robot children have to make a costly sacrifice: she must be completely dismantled in order to stop the bomb. Astro, having fallen in love with Nikki, stays with her till the very end. All that is left of her are her legs, which Astro takes to Dr. Elefun and asks him to switch his legs with hers, so that she could be a part of him forever. Elefun fulfills his request, and Astro happily takes off on more adventures. (Source: Wikipedia)