The YBM fare rules for international upgrades are arbitrary rules! Some of you continue to act as if they were divinely written upon stone tablets and carried down the mountain by Moses.
DL has some kind of issue going on here... I suspect the best way for them to solve the problem of unsold BE seats is to liberalize the sale fare rules the way CO and others have done. On CO, you do not need to have a 7 day stay, just a Saturday night stay for their BF sale fares.