Packages/engrams will decrypt in to existing gear only as confirmed on the stream and Weekly updates.
You're better off leaving ~10XP to your next Rank and then doing a quick patrol, or trading a bounty.
There is one way saving packages/engrams will benefit: it will increase your Cryptarch rank, which will net you updated light level Cryptarch packages if you decrypt a bunch after the update.
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I've also got most of my factions just a few xp off ranking them up, including the Gunsmith, Eris Morn, and the queen. I also kept all my iron banner and trials bounties that have 25 legendary marks. Means I can buy at least two legendary engrams when the update hits plus my bounty slots are filled on all 3 characters so I can rank up my guns and armour a bit quicker. Another tip is to head to the Court of Oryx and get your Antiquated Runes charged up as when reset hit you can jump down and hopefully get a 335 artefact! That's my tips for preparing for update.