I always always to remind myself which episodes "Metamorphosis" and "The Changeling" are. The titles are so generic they don't trigger any memories of the plots for me. The same goes for a lot of the TNG/VOY-style one-word titles.
I have to disagree that "The Changeling" is generic. From Wikipedia,
a changeling:
[...] is typically [...] the offspring of a fairy, troll, elf or other legendary creature that has been secretly left in the place of a human child.
While that's not a perfect metaphorical description of what happens to Nomad, since it's partly still the actual Nomad, the applicable idea in common is that it is not the real Nomad, but something in place of Nomad and purporting to be simply Nomad, however with developmental defects but also superior abilities. Kirk summarizes that in the episode, and it's always worked for me.