Is there a cheap buy up to F? This way you avoid the change fee, get SDC on a last seat available basis, and fly in F. Better than paying half the change fee when it's waived for upgrades (or upgrade to C+ - SDC for that is last seat availability too).
I just called and paid $10 more and got my friend C+ on the same itinerary. No change fee or anything. Agent just had to manually reissue.