http://www.mynissanleaf.com/viewtopic.p ... 74#p491774
It appears the batteries are located within the same space in the various MY Leafs. In fact the 2018 MY uses the same chassis dimensions as the older Leafs. So, the question becomes: "Why won't Nissan do the necessary engineering to fit the higher density batteries in the older models and offer the upgrade?" I have yet to read a plausible answer to this question from Nissan, only their dealers quoting Nissan's restrictive battery policy. or user speculation.
The most plausible is the engineering + homologation costs will be too high to justify given what price they would have to charge vs. the actual demand. Lots of people say they want _____ but will actually not follow through or will not follow through when they hear the cost. And, Nissan core business is selling cars, which they'd rather do than selling you multi-kilobuck battery upgrades.
Nissan is well aware (well, prior to knowing about the crap degradation on the 30 kWh packs) that some (many?) folks would rather pay for a 30 kWh pack than a 24, if they have to pay full price to replace their degraded old Leaf battery.
And given the 30 kWh pack is heavier, the max cargo + passenger weight allowed will likely have to go down on cars that weren't originally 30 kWh cars.