I'm editing a music video for a band, it all exported fine, uploaded and we were all happy with it. However, three weeks later they had the track altered slightly and wanted me to put the new version of the track on the video. By this time I had deleted all of the raw footage, so I imported the Pro Res 422 file into FCP, put the new track on the video and then exported it using the same settings (Pro Res 422, 1920x1080, Gamma Correction on Automatic. Except the newly exported version is darker than the previous version. Not sure what to do?
This must be a Gamma setting, if you're not transcoding anything there shouldn't be any color or gamma shifts occuring. You should try exporting with Gamma not set to Automatic, but the setting it had with the previous export. (I think it's 2.2 on Apple devices?)
Alternatively, there's 2 solutions that could save you a lot of time.
Firstly, you could render the video without 'recompressing all frames'. This will not transcode ProRes to ProRes, but will just write the video frame-by-frame. Since it's already a valid ProRes file it should turn out fine, and since you're not doing another render pass you'll have less chance of getting a setting wrong / a gamma shift.
The second alternative -and the fastest one-, is to open the video you have AND the new audio track in QuickTime 7. With the audio file activated, select all (cmd-A) and copy (cmd-C). Then activate the QT window of the video file, press cmd-J to open Properties, and paste (cmd-V). You should now see an extra audio track being added to the list. Delete the old audio track, and save (cmd-S).
Quicktime will now save this .mov with the new audio track in there, without having to re-render the whole video file, meaning there's no chance of a gamma shift. Also, this way won't write a new file to your disk, so it takes about 5 seconds to save the video with the new audio.