?The Changeling counts as a shapechanger, which renders it immune to polymorph and gives it disadvantage vs. moonbeam.

Can changelings use polymorph on themselves?

TLDR: If you're playing a changling from WGE, then you can cast polymorph on yourself just fine.

Does Truesight work on changelings?

To your point, it will be up to the DM, but in both editions there is a non-magical way to detect a changeling. Going away from RAW, as a DM, I would consider them shapeshifters so truesight would work, as would Moonbeam.