Shifting Ensemble tracks in Long-running shows

I'm always curious to see, in long running shows, how ensemble assignments can shift over time. For example, in PHANTOM at some point over the years, Don Attilo went from being doubled with Passarino to being doubled with the Auctioneer. A more recent, more minor example, is in HAMILTON, where the little cameo appearance of Sally Hemmings in "What did I Miss" has gone from being played by Woman 3 (who also plays the Bullet) to Woman 4 (Who also plays Hamilton's Mother), at least on the national tour. Anyone know how these kind of changes happen and why, or have any other examples?