I started reading Dune Messiah, I'm a little bit in so no spoilers, but these guys caught my attention with how they treat their shapeshifting. The changelings in the show treat shapeshifting like a tool to be used while these guys change between faces, bodies, and sexes like you and I change expression.
I'm writing a sci-fi story and I had the idea that most changelings act like in the show but that there is this group, a sect perhaps that acts like the Bene Tleilax in this regard.

The Face Dancers are indeed how I always play changelings.
Due to how they're made, and what they are, face dancers are even more mutable than changelings, but I do feel like as changelings would.beco.e more and more accepted, their changeing would become more and more of a social or convenience thing than an identity thing, untill they settled on a comfortable in-between.

It would certainly work in MLP, perhaps the easiest thing would be something along the lines of the Bene Tleilax type of shifting is a special learned discipline, a bit like a martial art maybe, as opposed to the usual, instinctive (ie, show canon) way ?