In D&D Beyond, I compared the Doppelganger monster's Shapechanger trait to the Changeling playable race's Change Appearance trait. While I think the two are very similar, some differences stood out. I wanted to see whether my interpretation is correct, since the Changeling's shapeshifting ability is written less "tightly" than the Doppelganger's.
The Changeling's Change Appearance trait says (Wayfinder's Guide to Eberron, p. 61; emphasis mine):
Change Appearance
As an action you can transform your appearance or revert to your natural form. You can't duplicate the appearance of a creature you've never seen, and you revert to your natural form if you die.
You decide what you look like, including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. You can make yourself appear as a member of another race, though none of your game statistics change. You also can't appear as a creature of a different size than you, and your basic shape stays the same. If you're bipedal, you can't use this trait to become quadrupedal, for instance. Your clothing and other equipment don't change in appearance, size, or shape to match your new form […]
What I take from this is that you can take the form of any bipedal creature shaped more or less like the changeling of Medium size (as Changelings are Medium) – so, say, a Mindflayer would be possible, but an Ostrich would not, as the body is too different. A Changeling cannot turn into a Gnome, since they have to remain Medium-sized.
There is no restriction as to the type of creature in the stat block, so any Medium human-structured aberration or monstrosity could be duplicated, as could undead such as mummies or ghouls. The comment on race doesn't appear to be so restrictive as to mean "You can only change race", since it says "creature" in multiple places, and not "race".
In contrast, the Doppelganger's Shapechanger trait says (Monster Manual, p. 82; emphasis mine):
Shapechanger. The doppelganger can use its action to polymorph into a Small or Medium humanoid it has seen or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
I take this to mean that it can change into anything humanoid specifically, but can be Small or Medium – so anything from a Goliath to a Gnome in size will work, but not a Mindflayer (whose type is aberration, not humanoid) or a mummy (undead, not humanoid).
A Changeling is more restricted in terms of size, and can only be Medium, but can be any substantially human-shaped creature, regardless of the creature type, while a Doppelganger can turn only into Humanoids, but those must be Small or Medium.
Is my reading of this correct? Or is there some errata or Crawford tweet/Sage Advice that changes this?
Best Answer
They are different abilities and they work slightly differently
Your interpretation is correct. A changeling PC has no restrictions on the creature type they can change into, only general body shape and size. Doppelgangers can change their size but are restricted to the humanoid creature type (but appendage arrangement is limitless1). Whether you think the humanoid restriction makes sense or not, it is clearly there and there is simply no way to otherwise interpret it. Similarly, the size differences are also clearly written and unambiguous.
It is clear that the Changeling PC's ability has slightly different capabilities than the Doppleganger monster. Note that these two abilities don't even have the same name and appear to have no connection to each other at all beyond beyond superficially similar.
The two abilities were written at different times and published considerably far apart for two different types of creatures (PCs and monsters). But that is not at all unusual, in fact it is more unusual for monsters to have exactly the same ability as a PC. Monsters follow many different rules and have many different abilities, some that are kind-of similar to PCs but not really (multiattack versus Extra Attack for example).
It might be worth noting that the PC version is based on the "change appearance" ability of the alter self spell. Whereas Shapechanger is ability that many monsters have and each version of it is slightly different. Though actually the Death Slaad has the exact same variation as the Doppelganger.
No further clarification has been made that I can find, but I also don't think any clarification is needed. There's simply no reason the abilities would be expected to be the same.
1 - Or limb-itless you might say.