WHOT/WHOT-FM in Youngstown, Ohio have been playing Top 40/CHR since the early 1960s. At the time the AM format moved to the FM, it was still the same ownership.
And Fred, despite your industry nudnik terminology "gotcha" ("there is no longer any such format as Top 40"), everyone discussing this seems to know what was meant by Top 40. So much for being an uber-industry guy. Insulation seems to be your worst enemy.
Yeah, I remember when Jerry Starr and crew did that one very nicely for Myron Jones. 1330, with 500 watts day and 1000 watts nights.
Another older one is WKAQ in Puerto Rico. Set up by Mike Joseph in 1968, it moved to FM in the 70's and is still in the top 3 or 4 stations (out of 125) in the market and still high energy CHR.
I am not sure if there is 100% format continuity, but WAPE in Jacksonville was born as an AM Top 40 around 1958 or 1959, and is now an FM CHR.
Others I can remember that go back to the 60's are
KDON (now KDON FM) Salinas, CA
KRBE (now FM) Houston
WPGC DC (1580 was Top 40 in about 1958, I think)