I love the ideas that are spilling out in this thread -- thanks everybody! I very occasionally change a word or two(usually not more) in a song to make it easier to sing. When I do, particularly if it's a song whose author is known and living, I try to tell listeners what I've done so they have a choice of learning it the original way or my way. An example is the song "Total Strangers" by English songwriter Alan Bell. I learned the song from David Jones, who sings the last line of the chorus the way Bell wrote it: "Our time apart will be short, and pass." That "short, comma, and pass" is absolute hell to sing, although it reads just fine. My friend Bob Clayton suggested I change the line to: "Our time apart will be sure to pass." It's a great improvement in the chorus's singability (and a fine chorus it is!), so I've done it -- always crediting Bob with the suggestion, and Alan Bell with the original.

Other lyric creep (and there is much in the songs I sing) is attributable mostly to middle age, not intention.