Warm is basically Very Soft AC. This format, under other names, is #6 in Miami (WFEZ), #2 in San Diego (KIFM) and always #1 in Tampa (WDUV). It has to be done just right to get the desired sound but not too soft or its audience will age out of the 25-54 demo.
Chicago is unique in having two Classic Hits/Oldies stations. There really isn't any large market that does. And some large markets, like Boston and Atlanta, have none. (Classic Hits/Oldies is different than WDRV in Chicago or WROR in Boston, which may call themselves Classic Hits but are really pop-leaning Classic Rock. They don't use jingles and don't play Madonna, Michael Jackson, Gloria Estefan, etc. They only play songs that were popular on Album Rock stations while also hitting the Top 40.)
On the other hand, WLS-FM isn't really that far behind WJMK. Until recently, WLS-FM had better ratings than WJMK. So I don't think Cumulus will switch the format at this time.