I have read that a changeling can transform into any creature they have seen, unless it's larger/smaller then their size and they can't change into something that has a different body shape than them. So no extra limbs or such.
So my question is, can a Changeling transform into a Tiefling for example? Seeing as they have horns and a tail. I would think the horn won't be a problem, but maybe the tail would.
In the Official Published Version, Yes
As per the "Shapechanger" racial trait description on page 18 of Eberron: Rising from the Last War:
It does not say anything about horns, trunks, tails or other features beyond the basic arrangement of limbs. Thus Centaurs definitely out. An Aarakocra may also be out with wings on their back, if wings count as limbs, and since they are in the place of arms on real world birds and bats, presumably they do. By the same token though, appendages such as tails, trunks, antlers, and horns are never considered "limbs" on real world creatures in common speech or zoology, even where they are prehensile and able to function similar to a limb (an elephants trunk would be a prime example). Limb is not a specific game term, so the normal English meaning applies.
So when it comes to horns or tails it seems to be a bit of a free for all. Of course things like a Minotaurs horns don't actually mechanically function as they would on a real Minotaur, who can attack with them, because mechanical traits and statistics don't change.
Note that you also can't change your height "so much that your size changes", so shifting to a "small" race like a gnome or a goblin is out, which would thus be the majority of player races you can not turn into. Whether you can turn into a non-player race is unclear.
The ability was substantially rewritten (and renamed) from the UA and Wayfinder's guide version. The equivalent line in the Eberron UA version was:
That clearly leaves room for a more expansive reading of what "basic shape" means than the official version, although the example is still about the arrangement of limbs. Since the rewrite was likely at least in part to make the ability as clear as possible, it seems entirely likely that the change of phrasing was specifically to address confusion on what was meant or specifically to be less stringent about letting changeling characters turn into Tabaxi, etc. since there is little obvious practical reason to have such limitations. In any case, let us all rejoice that the eventual published version of the race was a lot less ambiguous.
Note that I wrote this the day after the Eberron book came out, so please leave a comment if subsequent errata, sage advice, or tweets from the designers contradict me.