Singing a song is interpreting it. Unless you do it exactly the way you heard it, including instrumentation, verbal nuances, timing, EVERYTHING, you are subtly changing the song by singing it. In that way every singer makes a song his or her own by including it in their repertoire -- changing the lyrics is just a further step in this process, not something new and completely different from it.
So sometimes I will change a word or 2 in a song if I feel it makes the song better. Even if the songwriter is still alive. Without their permission. "Better" here may mean better suited to the audience (bowdlerizing songs for younger audiences, for instance), or aesthetically better to my ear, or whatever.