Well, I think it makes sense to carefully parse what Elon and JB said about this in the 2017/Q4 conference call:
Analyst: "Yes. Thank you. It sounds like from the letter that you could do more than 100,000 S and X in 2018, but you're constrained by the 18650s. And I'm just curious what would it take to see the 2170 cells in these vehicles?"
Elon: "Yeah."
Jeffrey B. Straubel: "Well, this is JB. It's something we've of course contemplated, but it's quite a large change to the architecture of the module and the battery pack overall. And while the 18650 supply is somewhat of a cap at about 100,000 units per year, even just a few months ago we didn't feel that expanding and making some long-term bets on expanding that supply with Panasonic in Japan was really the right risk. It's something we could consider, but right now we're pretty happy with that balance and it matches our other production capabilities and our other investments."
I believe JB would have mentioned it if the changes went beyond the battery pack. Changing the battery pack is of course still a major change, but it's still a
lot better than significant changes to the chassis - which would basically require a re-design and re-qualification of the whole car and would require
major tooling changes as well in Fremont, because chassis and interior dimensions would change as well.
Given that 2170 has lower mass per kWh, I believe their plan is to fit it into the form factor of the existing battery pack with very few changes required on the chassis and elsewhere (beyond battery pack integration and maybe the cooling compressor: increasing the cooling compressor's capacity would make sense anyway).
Plus Occam's Razor: I'm pretty sure they already knew that 2170 would fit when they picked 21700 as their form factor back in 2016. I mean, killing the 'easy' expansion path for the Model S/X would have been an unforced error of epic proportions...