Like with disguise self, changelings can alter their total height by 1 foot. So they could make both their legs 1 foot shorter, and that would work. In theory, they could even turn both their legs into little stubs, with a 1-inch-tall torso, as long as they elongate their neck to be about 5 feet long.
But what if a changeling wanted to take on the guise of a certain pirate, who’s missing a single leg? In other words, leave their total height the same and keep one leg the same length, but make the other leg into a little stub. That’s a change of more than 1 foot (erm, so to speak), but it isn’t explicitly a change to their height, since their remaining leg determines how tall they stand.
That much can also be asked about the spell, but changelings add on the twist that the alterations are actual physical changes. So is there any difference in the situations between changing the length of a leg, and changing the length of body parts that don’t contribute to height?
That is to say, is there anything stopping a changeling from growing out 50 feet of hair so the party can cut it off and use it as a rope? Or, similarly, causing their arm/fingers to become 50 feet long to manipulate items from the opposite side of a room? Or causing their tail to grow from 0 inches to 50 feet for no particular reason other than giving me another body part example for presumably the same rule?
And then, what about doing all of the above at the same time?