To add to Scott's answer...
One of the "Rural Radio" tools the Commission uses is seeing that the proposed community of license is "truly independent of the urbanized area," if the proposed TX site does violate the 50% coverage rule Scott mentioned above.
One of my former employers tried to move one of their stations from one rural town (population ~3000) to a tower in the nearest large city (population ~180k) but the proposed CoL was a small suburb, which would receive "first local service" on the edge of the 70 dBu contour.
I'm not sure of all the details, but the Commission required a showing that the proposed new CoL was "independent", and the licensee and their consultant decided they could not make such a showing, so they allowed the application to expire.
Roger on that. If you can't prove it, don't try it. Besides, sounds like that'd mean the rural town which was the present COL, wouldn't have that (maybe any) radio station serving it.
