i think Sparteye was referring to songs which can change lyrics,
not different songs with the same tune. it's more a poetry problem (rhythm) than a music one (tune).
so "House of the Rising Sun" and "O little town of Bethlehem" can both fit their first lines into 10 beats (and so on), hence the interchangeability.
"La bamba" and "Twist and shout" are the same tune, a pretty common thing in music that used to be a matter of respect but is now called nicking.
what was that song about a lemon tree that used the Police tune of "Wrapped around your finger"?
and what was it that Shakespeare said about there being nothing new under the sun?