1. Brother Cadfael: A career change can hardly get any more radical than going from crusading soldier to herbalist monk (with a sideline of detection).
2. Captain Arthur Hastings: From soldier in WWI to London detective (of sorts) to cattle rancher in "the Argentine".
3. Washington Black: From child slave on a sugarcane plantation to explorer to painter (supporting himself by working as a delivery boy) to scientist and scientific illustrator -- all in the space of barely 20 years.
4. Mr. Lawrence Todhunter: From philantropist and occasional literary columnist to murderer (which btw is not a spoiler -- it's the book's explicit premise).
5. Ed Hunter: From printer's apprentice / assistant to amateur detective to "carnie".
Miss Maud Silver: From governess to private investigator.