When it comes to arranging songs, and the "purity" of the original source, I've always liked a friend of mine's description of the "purity" of the source material:
"One night in Ireland, a couple hundred years ago, Da is in his cups, and hears a song he really likes. Being slightly tone deaf and incredibly drunk, he decides to join in, attempting to remember this wonderful new song. Thus, another version starts. He goes home to his family, staggering and singing all the while. Most of those along the way are just pissed off at his singing, but one or two hear the song, rally their sleepy minds around it, and 30 versions are born of the song on his way home.
Once home, he sings this song to his family. His family, none of whom are trained musically, all hear the tone-deaf slurrings in slightly different ways, and when they share this song that "Da heard somewhere, probably a pub", they are sending a 3rd version into the world, where the same process will happen over the entire village.
The same thing happens over, and over, and over again. Eventually, either the original singer will be able to push through one generally accepted version, or the village itself will settle on a more popular version, but before then, the other renditions will wend their way throughout the local area, spawning new copies as it goes."
For a modern version of this, listen to a song once that you aren't familiar with. Then, sing what you can remember of it for a while. Then, record what you're now singing. Then, listen to it compared to the original.
THAT is where "oral tradition" really comes from.
And THAT is why I feel no particular shame in arranging songs to suit the needs of the time, as long as we keep true to the spirit of the song.