Hey Larry

Well let's say you've got a great guitar solo except for one bad note - pitch correction would be the way to go correct that single note.. For Vocals, if you want to change a note or two in the melody . . . most of this software can tame "warble" or excessive tremolo - your ears have to be the judge of how good they are. Some instruments correct better than others. If you're thinking of changing a key - up or down 1 or 2 - it might work . . . . depends on the instrument, and I'm not sure how a voice would respond beyond a semi-tone or two.

Melodyne on their full version claims to be able to correct one note in a mix - never tried it.
I'd visit some software sites where your explanations would be fuller.

Good luck - Ian

Ian
My "Original Tunes" Site
My gene pool needs more chlorine.