A changeling can alter its appearance. It can conceal existing scars or tattoos, or add new ones. So the tattoo will exist, but the changeling can hide it.
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.
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.
If you want it to be able to grow a tail when it changes into a tiefling or tabaxi, it is your choice. It is also your choice if you want the tail to be prehensile and able to grab and hold stuff. You can appear as a member of another humanoid race which will include things like tails, frills, gills, wings, horns, etc.
Unlike ponies, changelings do not have cutie marks. Following their metamorphosis in To Where and Back Again - Part 2, changelings have varied coat colors and insect characteristics, such as Thorax and Pharynx's horns, and they no longer have fangs or holes in their bodies.
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.
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.
Changeling blood is green, unlike most other animals. Changelings, despite their thinly and seemingly fragile frame, are capable of surviving otherwise fatal injuries thanks to their magical abilities.
As for being tattooed, changelings can definitely be tattooed, and they can just as easily erase the tattoo with a moment's thought.
Changelings born from two changeling parents are born in their natural form: pale, slender, with colorless eyes and white hair.
Changelings reach maturity slightly faster than humans and live as long as humans do, typically about 100 years.
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.
Most often it was thought that fairies exchanged the children. In rare cases, the very elderly of the fairy people would be exchanged in the place of a human baby, so that the old fairy could live in comfort, being coddled by its human parents.
Changelings typically have large eyes with narrow pupils, ranging in color from white, to yellow, to black. Hair colors are similar: Light grey, platinum blonde, shades of silver, all the way to the darkest hair color being a light brown.
Speed: Changelings have a base speed of 30ft. Languages: You can speak, read, and write Common and Sylvan.
Some types of octopus, squid, and crustaceans have blue blood. Their blood contains a high concentration of copper. When copper mixes with oxygen, it gives their blood its blue color. Blue and red are not the only possible colors of blood — some animals bleed green.
Blue Blood is a deep, neutral, Caribbean aqua with a turquoise undertone.
Color. The color of human blood ranges from bright red when oxygenated to a darker red when deoxygenated.
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 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.
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.
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.
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.