In the original question Are Changelings immune to the Polymorph spell?, it was decided that because Changelings didn't have the Shapeshifter trait (though the answer should have said Shapechanger), the Polymorph spell would still work on them. Now, with the release of Eberron: Rising from the Last War, Changelings have the following trait:
Shapechanger. As an action, you can change your appearance and your voice. You determine the specifics of the changes, including your coloration, hair length, and sex. You can also adjust your height and weight, but not so much that your size changes. You can make yourself appear as a member of another race, though none of your game statistics change. You can't duplicate the appearance of a creature you've never seen, and you must adopt a form that has the same basic arrangement of limbs that you have. Your clothing and equipment aren't changed by this trait.
You stay in this new form until you use an action to revert to your true form or until you die.
And with Polymorph's description:
This spell transforms a creature that you can see within range into a new form. An unwilling creature must make a Wisdom saving throw to avoid the effect. The spell has no effect on a shapechanger or a creature with 0 hit points.
With Changelings actually having this trait, do they automatically pass Polymorph's saving throw and remain unaffected? This only pertaining to unwilling Changelings, of course.
Best Answer
Yes, it doesn't work on them, but not because of that ability.
Polymorph doesn't work on Shapechangers, and that doesn't refer to any creature with a shape changing ability, but to creatures with the Shapechanger subtype specifically - for instance, it will work on elf druids, because they're Humanoid (elf) rather than Humanoid (elf, shapechanger), even though they can use the Wild Shape class feature to turn into animals.
While it doesn't specify any subtypes for Changelings in their race writeup in Eberron: Rising from the Last War, in the antagonists chapter of that book, the example Changeling is listed as Humanoid (changeling, shapechanger), so they're immune to Polymorph effects if unwilling.