Speaking as a Leaf driver and an economist, I have to say that this transition from the current generation of EVs to the next is fascinating. It also makes me quite happy that I'm not in the room helping Nissan make decisions; I've been in product design and planning meetings outside the car industry, and the phrase "blood on the walls" isn't nearly as much of an exaggeration as one might presume.
I've read the InsideEVs piece (linked to earlier in this thread) speculating/reasoning about a longer range in the Leaf I coming this year. While I would love to see that and it's obvious that Nissan needs to maintain their leadership in non-luxury EVs, I'm not convinced it will happen. Nissan would only do that by using whatever new battery pack is in the 2016 and 2017 Leafs in the 2018 Leaf II. (Before I forget: I got a call from NMAC the other day offering me a six-month lease extension during which they'd only charge me for only four payments. I got the woman to tell me when the 2016 Leaf is arriving: September 7. She didn't know anything about battery range.)
I think there are still some intriguing possibilities here:
1. They put a new 2X range pack into all 2016 Leafs with no/minimal other changes, and when the Leaf II arrives, that battery carries over to the SV and SL, and the S reverts to the current pack. Seems highly unlikely; they'd have to reduce the range on the S going from 2017 to 2018.
2. They put the 2X pack into the 2016 SV and SL, leave the S range as is (but with a price cut), and when the Leaf II arrives, the batteries stay the same. This makes sense, as it lets them have an entry level car for those who can live with the current range, and they're only developing and producing/procuring one more battery design.
3. They put a 1.5X pack into all 2016/2017 Leafs, and then in the Leaf II the S stays the same (1.5X) and the SV and SL get bumped to a 2X pack. This seems unlikely to me, as it makes the product line complicated and it requires two new packs in short order, something Nissan would likely not be thrilled with, even if they could spread the development cost over the Leaf and their electric minivan (eNV2500?).
Besides the obvious move to increasing battery range, I think Nissan is feeling pressure to give owners at least some choice in range, and option 2 above seems to me like a reasonable way to get there.
I really doubt we'll see a retrofit of the longer range pack. Nissan certainly could make that happen, but I doubt they'll want to invest the development cost plus deal with the implementation hassles. Who would do the battery swap? Your local dealer?