I listened to a few stations that did the ole' format change prank, especially back before the days of the web and social media when the world was a smaller place and people perhaps a bit more gullible. One in particular was the #2 station in the market, and they switched from rock to oldies and kept up the joke 1/2 the day. The morning show jocks claimed they were so upset about it that they barricaded themselves in the studio and refused to leave at the end of their shift. They put several live callers on the air complaining bitterly, while a few said they liked the change. Finally the GM "broke into the studio" at sometime after noon and announced that it's well proven that everyone loves the oldies....And that everyone also loves a good prank, at which time they all yelled "April Foools!!' It was a smaller market, so I guess it created decent buzz and attention for part of the day.
One of the more creative I heard was when the morning hosts from all the stations in one particular market, regardless of format, all got on the phone with each other, shared playlists and nearly every station played the same songs at the same time. This was long before the internet and satellite radio, and back when every station for the most part had live staff during every daypart. People in their cars who only had radio for entertainment were confused as they flipped from one station to the other, both AM and FM and kept hearing the same music everywhere.