No, if you save faction packages they'll still decrypt into old stuff. It worked that way for TDB and for HoW as well.
What *does* work is pushing as many factions as you can to be close to ranking up. As soon as you've got 2,400 ish rep, you change class item and stick with another. Then with some saved up bounties, you can push all your factions to level up as soon as the expansion drops.
With HoW, what you might be confusing this with is the fact that people were farming Patrol for the Wolves public event to get Treasure Keys from those chests. The chests gave out blue and sometimes purple engrams which couldn't be decrypted because they were HoW stuff. So when HoW dropped we were full of engrams that we'd earned prior but that gave new gear. That and the fact that a day or two before the expansion dropped, Salvage started giving HoW weapons like the Matador as a reward.
But faction package rewards in the Postmaster will invariably give you HoW gear (or more likely some Motes
)