as a musician, I've seen far too many of these to begin to catalog.

that said, "Amazing Grace" (the hymn), the Eagles' "Peaceful Easy Feeling", the Gilligan's Island theme song, and the Animals' "House of the Rising Sun" can all be sung with interchangeable lyrics and melodies - i.e., the lyrics of "Amazing Grace" to the tune of the verses of Peaceful Easy Feeling, the lyrics of Gilligan's Island to the tune of House of the Rising Sun, etc.