Confirmed via a tweet by @ClancyElites, which was then retweeted by the official The Division Twitter account, it looks like stockpiling Exotic Caches until the release of update 1.8 is a very good idea!
Exotic Caches earned during play in 1.7 will still have a chance to drop 1.8 Exotics, once the patch is live.
We’d recommending holding onto your Exotic Caches, waiting until the 1.8 patch is live, and then opening them all up. You’ll then have a greater chance at grabbing one of the four new Exotic weapons!
— The Clancy Elites (@ClancyElites) November 14, 2017
As mentioned before, this tweet was retweeted by the official Twitter account, so it’s worth paying attention to. Resist the temptation to open your caches early, and you’ll likely be rewarded!