If you buy paper ticket from B6, you can refund it through them. If you buy ezeds, buy one leg at a time tkts so you can refund them as needed. ezeds usually need to be reissued online, then the unused one gets refunded but there are different situations. I had ezed MXP/FCO and could not get on, so I took a bus to other airport LIN to get to Rome. I booked it on the shuttle bus! AZ accepted the "old" tkt and I had the other refunded.
I always book single legs for convenience. Paper tkts can be "assigned" to different flt, but that varies, too.
Does that help? (or did I blonde it up too much?)