You're changing these attributes relative to your current appearance: as noted, changelings start out roughly humanoid, and none of these can remove the things that make a person not a skeleton.
Changelings can only shapeshift into other humanoid forms of the same size (humanoid size, in contrast with shapeshifters that can take up any size and appearance). Appearances they adopt are called disguises. They can swap any of their already possessed disguises at will.
The only other limitation is you cannot change your size category: as a Medium creature you cannot become a Large or Small creature - you can become a dwarf, you cannot become a gnome.
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.
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.
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.
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 born from two changeling parents are born in their natural form: pale, slender, with colorless eyes and white hair.
As for being tattooed, changelings can definitely be tattooed, and they can just as easily erase the tattoo with a moment's thought.
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.
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.
Therianthropy is the more general term for human-animal shifts, but it is rarely used in that capacity. It was also common for deities to transform mortals into animals and plants. Other terms for shapeshifters include metamorph, the Navajo skin-walker, mimic, and therianthrope.
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 are grotesque shape-shifting ponies who can morph their bodies into anything. These frightening creatures have fangs as well as insect features and translucent wings designed to scare even the bravest pony into submission.
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 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.
Their bones. Specifically their teeth. Cause while they're able to warp the rest of their body, their bones need to stay solid and ever present, so you cant change the pattern of your teeth (though maybe slightly the shape of your mouth. Dental records are important in dnd suddenly.
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.
DS9: "Chimera" suggests Changelings have a lifespan of at least two hundred years, although if Odo was indeed two hundred years old when he traveled back with the USS Defiant in an alternate timeline in DS9: "Children of Time", that would indicate Changelings could live for at least four hundred years.
In their true form, changelings are generally extremely pale, showing features devoid of distinctive details, with colorless eyes and silver-white hair.
Vocal Replication: All Changelings can mimic any sound they hears using their voice. Voice Manipulation: All Changelings can freely manipulate their voice, allowing them to control their voices to imitate sounds of creatures such as animal noises and explosions or increase or decrease the tone of their voice.
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.
Oxygenated (arterial) blood is bright red, while dexoygenated (venous) blood is dark reddish-purple. The difference is color results from the electronic state of the iron ion (ferrous vs ferric), which in turn influences the π → π* and n → π* electronic transitions of porphyrin and hence its optical characteristics.