I did something like that a few times and (at least for me) it´s was a huge amount of work and in the end it still sounded not really great.
You´d have to mix the voice out of the original (mostly the only mono thing in the song) and just leave the stereo parts. So the voice is much more quiet, although I wont disappear at all.
Then you have to at least make it to a difference of an octave (
, which would be easier than pitching it to the same from a 6-tone difference.
And then you have to change the speed of the voice (which you have to get the same way as just the instruments, only difference is you have to leave in the mono parts and cut out the stereo parts).
Though that still wouldnt sound very good, my version (two versions of a song called "Santa Maria") sounded like a choir of chipmonks in the end...lol