Due to the trait Shapechanger, changelings still possess the "shapechanger" keyword, thus are immune to polymorph.
WotC has confirmed that changelings are considered to have the Shapechanger subtype. So they are indeed immune to polymorph and can be affected by moonbeam.
You can't Polymorph a shapeshifter against their will. Shapechangers are immune to polymorph so that's one problem down.
No, they are not immune to polymorph spells. A caster can polymorph a changeling in to another form. However, since changelings are natural shapeshifters, they can immediately change their form on their next round of action.
Any creature that does not have a Challenge Rating or level cannot be targeted by polymorph.
While True Polymorph allows any creature, Polymorph only allows beasts, so this section will analyze available beast forms. Unfortunately, the highest CR beast is currently the Tyrannosaurus Rex at CR 8. There is no “best” form; every problem requires a different tool.
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech. The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.
Due to the trait Shapechanger, changelings still possess the "shapechanger" keyword, thus are immune to polymorph.
Can You Mutate Onto a Changeling? Sadly, a creature with the changeling ability will also have the “human” creature type, and you can only mutate on non-human creatures. So no, you can't mutate onto a changeling.
Will a legless changeling regrow both legs when it copies someone? I say no. The changeling can change their coloration, hair length, sex, height, and weight, but they can't change their number, arrangement, or function of their limbs, because nothing in the feature says they can or even hints at it.
A changeling shifts shapes the way others might change clothes. A changeling is a shapechanger. Yes, but statistically, the only parts of the race description that matter are those listed under the "Changeling Traits" heading.
Does Truesight Work on Changelings? Yes. Truesight specifically counters the shapechanging abilities of Changelings by revealing their true form. The Changeling is still in their altered shape, but the being knows what they actually look like.
Changelings cannot alter themselves enough to change physical sizes (i.e, changing from Medium to Small, etc). It is worth noting that while a changeling could technically shift into something like an aarakocra, their wings would be non-functional.
Appearance. In their true form, or their form "at rest", changelings have pale skin, colorless eyes that can be completely white or completely black, and white or silver-white hair.
Conversation. By RAW changelings revert when they DIE but not when they are unconscious (it would be hard to maintain a long term masquerade if you revert when you sleep!). I bet suggested that a piece of a Changeling — such as cut hair — WILL revert, but that this is slow and may take up to 24 hours.
Changeling Reproduction
If a changeling in a female form conceives a child, she loses the ability to change her gender until after the child is born.
A changeling can eliminate or add traditional physical disfigurements - such as scars or birthmarks - when changing shape.
Changelings reach maturity slightly faster than humans and live as long as humans do, typically about 100 years.
Polymorph turns a creature into another creature with CR equal to or less than the level or CR of the target, at least in 5e. The tarrasque has CR 30. No player character can reach any level higher than 20, because they don't exist. Therefore, you can't turn into a tarrasque using Polymorph.
Obviously, a changeling's appearance can change at any moment, but they do have a true form. Their true skin is usually grey or pale white, and their facial features are blank slates to an unnerving degree. They're typically thin and gaunt, and their hair is usually silvery with an occasional green or pink tint.
True Polymorph
With this spell you can become a second Tarrasque. The only way to fight a monster is with a monster of your own. You get all its abilities plus your allies can heal you and also you don't die when you drop to 0 hit points, you just become yourself again.
Polymorph B has a somewhat lower melting point and is dissolved much faster. Polymorph C is the most unstable form and is relatively rapidly converted to polymorph B, for example, during grinding [5].
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 becomes permanent.
No. An owlbear is a monstrosity, and a druid can only turn into beasts with Wild Shape.