OP - You are entitled to a refund of the fare difference, e.g. SM difference. Just submit the request for a refund. This isn't handled over the phone.

However, I would advise against it. If seats open in C+, you are already ticketed in C+ and only need to select (or call in to select) those seats. While six across won't likely open, if you could seat people in pairs, that might well happen in increments as time progresses.

When you return, you may then submit the refund request for the people who did not make it back to C+. Or just leave everybody together. Either way, I would not touch the tickets until you are back.