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Well that's kind of impossible since you get loot based on when you received the box. If you earn a box in 7.2, it drops 7.2 loot. If you earn it in 7.2.5, it drops 7.2.5 loot. They do this to stop people doing exactly this. Feeling as if they are forced to hoard boxs to keep up to date with the most current content.
"Note that this does not apply retroactively to Emissary Bags that you may already have in your inventory when the time comes. Only new bags earned after the Nighthold is open have the chance to reward an item level 940 Legendary."
https://us.battle.net/forums/en/wow/topic/20752570804#1
It's pretty safe to say this is going to be in place yet again.