Changing sequence settings to import 5.1 mix

I’m trying to import the completed 5.1 audio mix into my project, in order to replace the original unedited audio.

The original sequence was set to Stereo, and had dozens of audio tracks which I had exported for the post sound guys to work on. They completed my 5.1 mix, which I now have on six separate tracks. I read that I have to change the audio settings to Multichannel in order to set this up properly, but when I duplicate the original sequence it won’t let me change this part.
 

So, I tried to create a new sequence from scratch. I was worried about duplicating all of the original video settings, especially since there were already specific adjustments I had to get right to crop the footage into 4:3 (including a few cases of reframing). That’s why I would rather just duplicate it and adjust the audio afterwards. But I tried to create a brand new sequence, and set up the video settings manually. Unfortunately it isn’t letting me adjust the color management to match the original. I’m trying to make everything the same, and it appears like everything IS the same, except for the fact that I can’t adjust “Highlight Saturation.” The original sequence was set to 1.0, but the new sequence won’t switch from .5. I don’t know why the original sequence was set to 1.0 in the first place (my old assistant?) and to be honest I don’t really understand what it means. Again, I think it has let me change everything else, so I don’t see another difference that would clearly explain this limitation.
 

So, at the moment I seem to be caught between not being able to just duplicate and adjust the audio settings, and creating a new sequence with the right video settings. What can I do? What’s the simplest way to import the complete professional audio mix without messing with anything else?