Editing pitch from a video

I recently shot a video that included someone singing to a background of music. As I view the video, some of the voice notes are out of tune. Since it is a .MOV file, I believe I could tackle the job of trying to figure out how to upload this video into Sonar. I could then use Melodyne to correct the pitch. HOWEVER, I would think that if I raise or lower pitch in Melodyne, I'd be doing the same to the musical accompaniment--which is already in perfect tune, making it sound potentially worse. 
Has anyone tried this? Perhaps if I just change things for 1-2 seconds here and there, the instrumental part change would not be that noticeable. I do notice the abnormal vocal pitch IS very noticeable. 
While writing this, I can remember in the past using my joy stick to pitch shift a guitar type patch. Sometimes, instead of just the guitar patch being affected, several tracks were changed as I'd done this. That was VERY  noticeable, though, to be fair, I usually will pitch shift a whole note (2 half steps) when playing a guitar patch like this. I would think I wouldn't be pitch shifting with the voice more than a half step at a time.
Any thoughts? I tend to think the answer will be something like:  :Don't do this."
Thanks.
LNovik

Theoretically is the key. I can't imagine i could do a good job of it. Certainly not a transparent job.

Thanks for those replies.
Song said: Theoretically, if the backing instruments are in tune they shouldn't be affected. You'll need a polyphonic version of Melodyne
The first part of your sentence confuses me: if the backing instruments are in tune, they shouldn't be affected. If I am, ie, raising the pitch of a sung note by 1/2 of a step, won't the  backing instrument for that 1 or 2 seconds also be raised?
That brings me to the 2nd part of your sentence. I do NOT have Studio or Editor version; I had just updated to another version--I believe it's called Assistant-- but not either of those.
SO, perhaps you are saying that I COULD do this Only if I had 1 of these polyphonic versions.
LNovik
 

LENovik
SO, perhaps you are saying that I COULD do this Only if I had 1 of these polyphonic versions.
LNovik
 

It is possible to adjust each instrument individually with the Editor or Studio version, but it could take a lot of work. Celemony have a demo video in which they alter the vocal melody of an Adele track despite only having a stereo mix to work with. It's impressive, but there isn't too much going on and they have a perfect recording to use as a starting point.
 
This video shows the basics of the procedure...
Celemony Video
 

I used Melodyne to remove a 'ding' caused by a vocalist kicking the mic stand during a take. It worked a treat. (I now have a wall mounted, isolated mic stand) :-)