There have been some hints that changelings might actually be shards of a certain mutable God, actually. It's never been explicitly defined, but to me that possibility does give changelings quite a specific fit into the world (though them then saying "I am an Elfen and thus currently a shard of Elfenehoala!" doesn't fit so well at that point).Lerad said:Reincarnation and changelings aren't mentioned in the histories because they do not really fit ICly. At least, that's what I think. And it's pretty much a fact that these are mechanics that are put in out of neccessity - the game needs to give players the option of changing their race, and they have a premium artifact for it. And there's no real way to explain this to 100% satisfaction in the lore. At least, I've never seen it explained, it's only ever glossed over.
Answers
An ic theory about racechanging, etc, that @Kiradawea should get credit for (unless I am completely misrepresenting it, in which case, sorry) is that the arrival of humanity has dramatically affected how shards can interact with each other and with the portal. Prior to their arrival, interbreeding was canonically impossible (I think) so it's unlikely that getting "miswoven" was a regular thing (except maybe for faelings/elfen). With humans getting their amorphous non-templated souls mixed in with the more rigid shards of the templated elders, mistakes happen, souls maybe can be more than one shard and people identify more stringently with one or another as they develop over time.
My impression of the tapestry is that everyone is a part of it, but going through the portal gives agency within the tapestry. You were already part of the tapestry, like Billa of Estelbar, but now your thread is specially coloured and you get to choose how you are woven, unlike Billa. (Which some people have suggested explains why most inhabitants of the basin are very repetitive)
I know there's some player speculative scholarly works out there about some of this with others being written (some of which are pretty outdated with theories that various elder wars have disproved, but it's still fun to read) and in Hallifax there are people who will occasionally start randomly theorising over the city aether.
I don't think there's an explicit divine-given answer anywhere. Which just leaves more room for us to say ic "I have no understanding of how this works, but we know x, y and z so maybe this theory explains it!" or to just shrug and go "It works, I don't really care how."