It's not just those two examples, but has happened a number of times here in Philadephia, and I suspect elsewhere.
In the late 60s, the old WFIL-FM was a soft-rock station. Once sold and changed to WIOQ, the station morphed into a progressive rock format, by way of harder top-40 stuff.
WMGK started in 1975 as pretty sleepy, near background station playing Ian and Sylvia mixed with popular John Denver tracks. By the early 80s, it had added harder, more popular songs, from acts like Styx and dumped the folkie stuff. By the 90s it went harder again by become a edgier 70s station, then to Classic Hits, and finally full-fledged Classic Rock.
WDVR was the sleepiest of all sleepy stations, later renamed WEAZ EZ-101 while adding some vocal tracks, then it competed directly with MGK, SNI, and Kiss 100, by dumping the instrumentals. It's way "harder" than the station used to be.
Heck even WYSP stood for "Your Station In Philadelphia" with its beautiful music format, but wasted no time going right to progressive rock.
Down where I am in Florida, a Tampa station known as "the Dove" played old "magic" style stuff when I moved down here. The older population ate it up and regularly gave them double-digit ratings, trouncing the market with the most semi-warm bodies. Over the last five years or so, out went the sleepier songs, and in came the harder stuff to the point where the dentists' offices and car shops complained. The numbers are now lower - I suspect the demo is a bit too.