I would say (without extensive research) Classic Hits will get there before Classic Rock. WLS seems to be pushing up the years and staying there, reducing 70's tracks, increasing the 80's, and dipping their toes in the 90's. WDRV will start to dabble with grunge and a couple other artists here and there, and then back off. Then again, my FM listening has dwindled over the last few months, listening to Sirius XM and my own library a lot more. Classic Hits is/will be a more flexible format overall to evolve, while Classic Rock has become pigeonholed into a cwrtain style/sound, for better or worse.
Re: Moving Through Time Classic Rock & Classic Hits
I think it's a generation gap more than anything. Although 97.1 FM The Drive will occasionally play early tracks from The Rolling Stones, Jimi Hendrix, The Who & The Beatles, a lot of their playlist tends to be from 1970 through about 1995.
The Drive also seems to have sprinkled in quite a bit of '90s alternative rock from Chicago's own Smashing Pumpkins, Pearl Jam, Stone Temple Pilots, Nirvana & Metallica, particularly from 1991's self-titled "Black Album". What came out in 1994, for example, is now 30 years old.
Along those same lines, oldies stations have also added quite a few '80s tracks, too. Once again, it's a generation gap more than anything. Wanna feel old? A song that came out in 1984 is now 40 years old! Not to reveal my own age, mind you...