I'm not aware of any programs that do this, Mick. I believe 'Melodyne' can, somehow, get into and alter components in a stereo mix. Perhaps you could filter the top line out with that. I imagine it costs a few bob, though. I've never had my mitts on it.
I also imagine you could be entering the realm of Chinese Whispers, where, potentially, you put in, 'She Thinks I Still Care' and end up with the musical equivalent of 'She Stinks, I Wear Flares'
Good luck with the task and please let us know how you get on.