If you are flying ATL-JNB on Saturdays and JNB-ATL on Fridays it is almost impossible (I am sure someone is going to chime in saying not so ) to upgrade from B/M fares using SWU. The only reason I still fly Delta is due to that route and I do it often enough that I have seen the pattern.

In my opinion paying $6000 for a lie flat seat that has a width of a coffin is not worth it. Nor is paying for B/M fares knowing that it will never ever clear on those flight days. Just get a U fare ticket on that route and enjoy the Delta hospitality of running out of water/soda/juice midway through the 16 hr flight...

Yes, front is full on those flights and mostly are paid B/E travelers along with a bunch of non revs. I have noticed sometimes the purser comes along and calls out for passengers and move them up front after the doors have closed. I guess they were Delta employees or "optimists" who purchased B/M class fares. But they happen only on mid-week flights.