It may be possible to "split" the booking, putting the 2 passengers who are no longer travelling onto a new booking reference, and then cancel it.

I have split a commercial booking before to allow a change of flight time for one person on the group, but not with a reward booking.

Maybe give your local service centre/line a call and see if they can split it for you?

Do bare in mind, with the snow disruption, it may take a while to get through if you do go down that route.

BBB