Changelings are able to change their form at will without expending a spell slot or using a disguise kit, but there are limitations to their abilities. For instance, they cannot change the clothing or equipment that they are wearing. Whatever they were wearing as their previous form will carry over to their new one.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
While changelings can look like anyone, they do have a true form. Their natural look can be scary to some due to their lack of detail and distinctive features. Their skin tone is always pale, either white or light gray, and they tend to have slender bodies with limbs slightly longer in proportion to other humanoids.
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.
Changelings are not transformed by magic since its an innate ability, that doesn't use magic.
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.
Changelings don't inherently recognize each other. If they don't want to be recognized then when they do something suspicious it would be an insight check against the changelings deception, which the changeling has advantage on. A changeling could have expertise in insight from divergent persona.
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.
Changelings have red blood like a majority of the other races. Bleeding around others may alert many that the Changeling is not of the same race (should the blood color vary). For this reason, a Changeling will usually choose to take the form of a race with red blood.
Some stories tell of changelings who forget they are not human and proceed to live a human life. Changelings who do not forget, however, in some stories return to their fairy family, possibly leaving the human family without warning. The human child that was taken may often stay with the fairy family forever.
Changeling Traits
Age: Changelings reach adulthood at the age of 16. Their fey heritage allows them to live much longer than humans, up to 200 years.
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.
Changelings were fairies who had been left in place of a human child or baby who had been stolen by the fairies. The child was taken for one of three reasons; to act as a servant, for the fairies to receive the love of a human child, or for malice/revenge.
As for being tattooed, changelings can definitely be tattooed, and they can just as easily erase the tattoo with a moment's thought.
Changelings are thought to be an uncommon race—though their true demographics remain unknown, as may live their entire lives masking as other sentient races. Many places in the world fear changelings for their unique ability to assume the form, or 'profile', of nearly any person, indefinitely.
The short version is that a Changeling can shapeshift to a form that can produce sperm to impregnate someone, and can shapeshift to a form with a uterus and eggs to get pregnant, but a pregnant Changeling cannot shapechange to a form without a uterus without ending the pregnancy.
Changelings have an interesting relationship with gender as a concept, which is probably part of the reason a lot of the art doesn't find it necessary to distinguish the males with facial hair. As they're changelings though, they can have any kind of hair they want.
Changelings do grow physically, to a point; as Changelings don't eat or drink, it is unclear exactly where the 'new' mass and energy come from.
Powers & Abilities
In their human forms, the Changelings also appear to be immune to the effect of sunlight just like Stalklings, regardless if they are in their troll forms as well, thus making them the perfect spies for the Gumm-Gumms.
17. As a playable race, changelings have the following abilities: Shapechanger: Using an action they can change their appearance and voice at will, even adapting their size between small and medium in some cases.
Changelings lay their eggs directly within food sources her offspring will eat after hatching - namely stashes of meat - that the mother has procured and hidden out of sight in a secure place, such as the abandoned burrow of some small animal, a crevice in rock, or a small hollow in a tree.
Unlike doppelgangers, which are entirely genderless, a changeling does have a default gender that manifests in adolescence, but each changeling can adapt his or her form to be of either gender, hermaphroditic (both genders simultaneously), or entirely genderless.
changeling, in European folklore, a deformed or imbecilic offspring of fairies or elves substituted by them surreptitiously for a human infant. According to legend, the abducted human children are given to the devil or used to strengthen fairy stock.
Changelings mature slightly faster than humans but share a similar lifespan—typically a century or less. While a changeling can transform to conceal their age, the effects of aging affect them similarly to humans.