Not technically Top 40 (although this source says it was), but it was a big shock for people who liked the traditional sound when they changed in 1971.
This is fascinating. it looks like something that would happen today. WBT was number nine, which wouldn't be a big deal today. it used to be number one and would be again/
http://www.reelradio.com/ideas/index.html#wbtgmrh