The Polymorph spell says:
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.
Changelings (from the original version of Wayfinder's Guide to Eberron, as they appear in UA: Eberron Races), which are similar to doppelgangers, are able to change appearance, but don't seem to be explicitly shapechangers.
Are UA changelings actual shapechangers and therefore immune to Polymorph?