doesn't מנצח also mean winner? I didn't answer that way; I'm just curious.
Yes, it does.
Does this mean he decided to change the key the piece was being played in, or that he came up with a different scale altogether?
How to make the orchestra really hate you!
הַמְּנַצֵּחַ הֶחְלִיט לְשַׁנּוֹת אֶת הַסּוּלָּם.