Thanks for the replies. Technically, BD don't allow any changes within 7 days of departure - sometimes it is possible, but could be a problem with the 3-7 day pre-departure window for releasing more seats. That's why I was hoping to find a second seat now if possible.

Unfortunately, no 773s available - just 772s that day.

I guess I'll probably just go for a C seat on the same flight, and hope for the best...