Newly created channels in instruments.xml not showing up for old scores.

I added a "solo" channel into instruments.xml for violins to be able to switch from tutti to solo multiple times (I would prefer not to change instruments because it creates a little bit of a mess in the mixer panel that makes applying a different soundfont a nightmare). However, on my score that was created before I updated instruments.xml, the new channel is not showing up. It does, however, show up when I create a new score. How do I get this to work? Thank you.