Leonard Cohen once said that range is for backing singers.

(I think he meant it as more of a self-deprecating joke than anything imperious)

Anyway, I don't see anything wrong with altering a song to suit your own voice.

I've just been working on a version of 'The Grey Cock', as transcribed in the newly reprinted Penguin Folk Songs book, and I'm "cheating" slightly with the tune. I'm sticking to the notes of the tune as written, but there's a couple of phrases where the melody descends too low for my voice, so I'm instead ascending to sing them an octave higher. I think subtly altering a song because of a practical necessity is a much more organic way to approach it than, say, merely jazzing a song up for the hell of it.