Most residents of such a town fight vociferously to keep it that way or move out if they couldn't.
Who does anything so silly?
Sure it is. Except, the move isn't usually to an 'upscale' part of town (which are often exclusively white outside of the Northeast or CA), just one less populated by the urban poor. It takes place so often there is no need to comment on it.
Gentrification moves the population of the urban poor, or urban middle class, out, so it's this phenomenon in reverse.
Residents from the declining midwestern towns moved from nearly all white suburbs to nearly all white suburbs of TX and GA, and carried their bigotry with them.
Of course, as if you could reduce anything progressives do to a slogan. If you
must reduce it, then a good proxy would be "I can't be racist (or anti-black) because I moved from a place that has no races (blacks) to a place that has many races (and few blacks)." This is simplistic enough.
Not through anything as crude as stuffing "Get Out Before It's Too Late" flyers in mailboxes. But the effect of the urban renewal, and of the economic revitalization focused nearly completely on providing the amenities of an upper class (in SF, Manhattan, DC, etc.), or development that completely misses most of the traditional middle classes, effects a black evacuation.
The entire phenomenon of suburbanization from the 1940s was considered to be at base an example of federally mandated white flight. I say that, in the decades before the 1970s, white flight certainly was a strong factor. But white flight wasn't the only one: selective urban renewal that demolished black tenements, the placement of coming interstate highway system, redlining and zoning of utilities and sewage treatment facilities near the new ghettoes, etc. But all of these were considered progressive in the era, since the focus of down redevelopment was to transport suburbanites back to downtown as expeditiously as possible.
In the South, besides those things, the primary factors for white flight came after the 70s, particularly with school desegregation and the rise of black mayorships in the traditional cities.
But I maintain that reducing the trend towards suburbanization as a consequence of simple racism past the 80s is dishonest. Really, when would a family of five prefer to live in a rotting two bedroom tenament than a new tract house in a suburb? Even white parents who tolerated desegregation didn't want to subsequently bus their child across town to enforce it. These choices came irrespective of the
demograpics, since blacks who could own a home readily did so ... from their side of the red line.
I'd contend that the phenomenon of gentrification -- building a wealthy conclave with the sole purpose to raise nearby rents and drive away current tenants -- is clearly class-based, and, coincidentally, racially motivated. It's a reverse blockbusting.