[T]he population of modern Sichuan has only weak demographic connections to classical Sichuan, as instability in the 17th century resulted in a population crash to around ~1 million. Subsequent to this over 10 million Han Chinese from the regions directly to the east, Hunan and Hubei, migrated into the region, replenishing its population. This obviously has cultural and genetic implications….(if this was common, as some have asserted, then the low between population differences between Han regions in terms of genetics makes a lot of sense).Modern anthropologists and historians tend to have a hard time knowing what to make of extreme population turnovers that ancient DNA evidence makes clear have happened in multiple places at multiple times over the course of history. The event in Sichuan, in the historical era, could shed some valuable light on what this kind of population turnover event looks like in more human terms.
There are also some places in Europe where I understand that there has been great population turnover in the last thousand years or so: Hungary and some coastal and island areas around the Balkans immediately come to mind.