Try this, a parallel were benign alien life forms have landed on Earth and merged with the terrestrial ecosystem. Bazaar mutations have become the norm. Most people shapeshift into a new form if seriously injured. Many people undergo the same sort of event in old age or if they become seriously ill. The process resembles
Regeneration in Doctor Who. The main differences are that A) after regeneration the regererated individual's physical age is either in their early twenties, or whatever age they were before if they were younger that that, B) sex changes randomly {about 45% male, 45% female, 10%
intersex}, and C) the individual generally still looks like a member of their family, but within that limit this varies wildly.
Centrum, Homeline, and the Cabal, all see the advantages of long lived shapeshiting agents.