Hate to point this out, but in just about all of the stations mentioned, there are gaps in the station's histories where the station wasn't CHR.
An example is KDWB. There was a period of time from 1980 to 1984 where they were Album Rock and was no longer Top 40 by any means. That was why WLOL did very well in the market from the time when they've signed on in 1981, until they were sold by Emmis to Minnesota Public Radio in 1991. It took several years for KDWB to shake off the AOR image that they've made for themselves after going back to CHR (anyone remember "The New KDWB").
WAPE (AM 690) WAS Top 40 until 1981 when they went....*gasp*...Country, then....*gasp*(again)...Religion. The Kaplans still owned them at that point I believe and they were trying to get the 99.9 FM allocation for Palatka, FL so that they would finally have an FM sister for WAPE. Sadly, the deal fell through and WAPE was eventually sold to Statewide Broadcasting, which acquired the 95.1 FM frequency from the city of Jacksonville and WAPE-FM finally became a reality in early 1986.
WPGC-FM went downhill as a Top 40 in the early 80s and flipped to AC as WCLY (Classy 95) in 1984. The AM was doing Business talk, but wisely kept the WPGC call letters. The station was eventually sold to Cook Outlet Broadcasting, which brought back WPGC-FM in 1987 as a Dance/Urban Contemporary station. More info can be found here:www.amandfmmorningside.com
KRBE-FM abandoned Top 40 in 1981 for AC and didn't come back until 1985.
Y-100 abandoned Top 40 in 1991 for AC and didn't revert back until 1995.
The only 2 stations that I can think of that deserves the title as oldest continually Top 40 in the country is:
WABB/Mobile, AL (Started on AM in the late 50's, then went to FM in 1981)
WNOK/Columbia, SC (Started on AM in the early 60's, then went to FM in 1975)