Question: A changeling's Change Appearance states:

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> 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.
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But seeing that with a druid's Wild Shape can change you into a creature that is quadrupedal and a differerent size than yourself, does this mean that after that you can change into another quadrupedal creature of that same size?

Let's say you are a normal 2 legged medium changeling and you Wild Shape into a 4 legged large [saber-toothed tiger](https://www.dndbeyond.com/monsters/saber-toothed-tiger). Does that mean that you could use the Change Appearance feature of the changeling to change into another 4 legged walking large creature like a [dire wolf](https://www.dndbeyond.com/monsters/dire-wolf)? Could you also Change Appearance into a creature outside of the possible druid Wild Shapes? For example a creature with a higher CR?

Answer: 
According to RAW, you can't Change Appearance while Wild Shaped
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The rules for Wild Shape states concerning racial features is:

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> You retain the benefit of any features from your class, race, or other source and can use them *if the new form is physically capable of doing so*. However, you can’t use any of your special senses, such as darkvision, unless your new form also has that sense. [Emphasis mine]
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The crux is the highlighted clause: The creature assumed by Wild Shape is usually not physically capable of changing its appearance in the same way a Changeling can. This would mean the racial ability can't be used simultaneously with Wild Shape.
I would argue that there is no restriction from a Changeling using Shapechanger while in Wild Shape.
If the verbiage that is italicized above is relevant, then it would also prevent the druid from using a second use of Wild Shape while in a wild shape form, and it has already been clarified that this is possible:
[https://rpg.stackexchange.com/questions/96375/can-a-druid-wildshape-again-whilst-still-in-animal-form#:~:text=A%20druid%20can%20Wild%20Shape%20to%20a%20new%20form%20without%20reverting](https://rpg.stackexchange.com/questions/96375/can-a-druid-wildshape-again-whilst-still-in-animal-form#:%7E:text=A%20druid%20can%20Wild%20Shape%20to%20a%20new%20form%20without%20reverting).

Also, Keith Baker (who created Eberron) speaks about a Changeling Druid here: <http://keith-baker.com/faq-changelings/>