Tatie Bogle said
'I can think of a couple of songs where someone has decided to write a completely new tune to a well-known song - maybe they didn't know the original, or maybe they just thought theirs was better - not necessarily so.'
I am a dancer and choir member. The dancing masters of the past published dances with tunes, so now people think that dance and tune belong together. Not so. Another publisher might put the same instructions to a different tune, or a different set of instructions to the same tune. The dance usually went by the name of the tune it was set to.
Then in choir we sing so many different composers tunes for the same (usually religious) text.
So you could say that is the traditional way of doing things.