[RPG] Are UA Changelings immune to the Polymorph spell

The wording quoted in the question is from an earlier print of the PHB. In later printings, and in the SRD (v5.1, p.188) it reads:

The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation lasts until it is dispelled

This wording seems to answer the question, that "lasts until it is dispelled" replaces the ending conditions given earlier, but also notes that the effect can be dispelled.

For some reason, this change was never listed in official errata, though the wording did indeed change between printings.

Also, apparently Crawford stated explicitly that this clarification was made on Twitter.

No, the druid does not revert.

A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can't assume a different form until it leaves the spell's light.

As this is a new paragraph separate from the first paragraph of the spell description, and the paragraph exists to describe the specifics of what happens to a shapechanger, the "it" in the quoted passage can only refer to the shapechanger mentioned in the previous sentence.

Most creatures don't change out of their original form, so it wouldn't make sense to refer specifically to shapechangers in the opening sentence of the paragraph then clarify that all creatures return to their original form.