Well. For one, as DM I would not permit a thri-kreen...assuming the changling had seen one in the first place. It is not a "humanoid." It is an insect. Yes, I know the term 'humanoid" is a creature type and the thri-kreen are, in fact, listed in the MM as "humanoid (thri-kreen)", but I would say/rule the creature's additional limbs is a step too far to their abilities as, obviously, intended.
Aarakocra, I would think the more likely issue is the Changeling will not have seen one. And, similarly, is not a humanoid. Aarakocra in 5e have separate wings. Their arms are not wings, as in former editions. So, the Changeling could not, actually, make themselves an aarakocra since (as with the thri-kreen) they can not give themselves additional limbs.
The Merfolk, I have no issue with them doing that to be able to move/survive in an aquatic environment. Seems a perfectly reasonable and expected use of the power. However, again, the question then becomes, "When/where did your character see a merman [aquatic elf, locatha, etc...] to emulate?"
The answer to any of those "you have seen" questions, for a game I were DMing would be "if it's not a common humanoid type [as one would see walking around town]" that if it hasn't happened "in play", then it hasn't happened.
I don't think "saw it in a book" is sufficient for this ability, as written or intended, hence the "have seen" stipulation. So, for a "typical/standard" [if such a thing exists] D&D world stuff like humans, elves [the presence of drow would be an individual table call, mine would be a no], dwarves, halflings. Perhaps gnomes, half-elves, dragonborn, tieflings. Depending on the area the character is from: lizardmen, orcs, hobgoblins, half-orcs....are warforged medium or large? Goblins are "small" so they can't do that, since it has to be a creature "your size" and the changeling's size is medium, right?
Seems like the power is/would be significantly more useful in urban/intrigue/populated-type campaigns than dungeon delving.
So...yeah. I don't see it as being OP. In fact, as read, it could be U[nder]P if one were to interpret the "that you've seen" part to mean you can only become/change into a particular person/individual and not the type of creature they are. Which, to me, doesn't make a whole lot of [flavor] sense and they should definitely clear up the final language on that to make it clear they are talking about a species and not individuals only.