Can't say it went from THE best to THE worst, but WKSG 102.7 "Solid Gold Kiss FM" Mt Clemens MI (a Detroit rim-shotter - today WPZR) comes to mind. They had put together a pretty good Oldies format with live DJs, blowing 500w AM daytimer WHND out of the water, and were fun to listen to (they had neat features like Kiss Comparison - songs by two different artists. The playlist was rich, novelty songs were not off limits).
Come late December, they became "Oldies 102.7", with taped announcements, and a much narrower playlist. Now they sounded just like WOMC 104.3, which just two weeks earlier, had become "Oldies 104.3", with the same playlist, and the same announcement tapes!
WKSG had a TL in the NE corner of the metro area with 50 kW and a DA nulling the southwest.
WOMC had a TL in the middle of the metro area with 190 kW omnidirectional.
One guess who won this ratings war.