'Red state' Republican counties that became Democratic ones

[Unelectable Bystander]

The vast majority of these are counties that house a somewhat major city. Does anybody have any data on whether these flips are always a net positive for Dems in the context of the greater metro? For example, Cobb/Gwinnett etc certainly are because the entire ATL metro is moving left. Are any of these cases where Dems are flipping the county but not actually netting more votes due to offsetting migration?

One possible example that come to mind is DFW last decade. Right when Dallas county flipped, it looked like part of the reason was that republicans were just moving further out towards Denton and Collin, and thus netting more votes there. Maybe another one would be the Detroit suburbs flipping due to minority migration from Detroit itself, which is in turn shrinking. But I don’t have data to say those are conclusively true.