Seems like it will be. But Tesla has been really cagey about actually doing battery swaps. I hope they get more free with this in the future. For instance, what if I want to swap in a 90kWh pack into my 75kWh car that someone sold due to buying a 100kWh pack? I'd then want my 75kWh pack to go someplace: either I put it in a conversion kit to work as a time shift for my solar panels at home, or I send it back to Tesla for THEM to re-integrate those batteries into a time shift fixed battery storage product. I'd like to see much more liquidity in this type of activity. So far, it's been pretty damn lockstep. Part of the problem is that the fixed storage business is pretty young, and supply constrained, and the cars are supply constrained too. You'd think this would allow shifting used batteries to the supply-constrained marketplaces as long as someone is interested in buying a new upgrade, but it is SO supply constrained that Tesla doesn't even NEED to service these upgrades, since all they have to do is force you to trade up a whole new car or fixed storage, and just not worry about it AT ALL -- it's a seller's market right now.
We need to get a bunch of 90kWh owners who want to pay a huge huge huge amount of money for a few miles more and less than a second of track speed for this marketplace to really open up in any real way, and I'm not expecting that to come to fruition, until there is a much healthier mix in the marketplace that doesn't jam up all the possibilities since there's no movement just yet.
I think what will unjam this is GigaFactory #1 being online in a big way: once they start making more than enough packs for Tesla Energy (fixed storage) AND Tesla car new production and upgrades (for Model 3, Model S, Model X, etc.), then the chicken-egg question will be: can I upgrade my ___ (fill in the blank) to a ____ (fill in the blank), and if the Gigafactory can (1) make the upgraded pack (yes) AND (2) recycle the old pack into a fixed storage product that someone is willing to buy even though it contains used cells (yes), then suddenly this type of activity will become more frequently possible. I'd love to work inside Tesla to do the logistics programming to make this sort of thing possible, but I bet I'd be railroaded for a while before I could catch up to whatever logistics system they're using and be able to figure out a way to shoehorn this into their current process flows in any sort of smoothly integrated way.
In effect, my guess is that they will eventually make this possible, sometime after Model 3 has been humming down the production line for a while. That's about when the Gigafactory #1 will be proficient enough that these possibilities will actually be possible. Throw in the demand from many of the owners, and I don't think it will take much for these prices to start coming out. For some odd reason, it's not an easy swap; the various generations of revisions of the Model S & X line is pretty substantial, so there has to be a pretty proper specialist in there to do your battery swap for your particular car to the particular new pack being installed. While that can all be figured out back at the mothership by the designers and engineers, right now, they're pretty occupied with the new models (Model 3, Y, truck, bus, etc.) being put to market. Once they finally have a breather from that, this "clean up" stuff, like these types of upgrade options, a healthy used maintenance marketplace for out of warranty Model S & X's, and Model S & X redesigns (v2? chasing the mature competition luxury appointments, at least a step or two?), will have to be properly developed.
So ... 2019? That's my estimate. Could be 2017 if Elon makes a point of doing it, or 2021 if Tesla just wants to do least-effort. And it could be done partially by 2017 (which would seriously open up this marketplace liquidity) if my suggestion above was taken, that owners who want to upgrade like this must also take delivery of a "conversion" kit to plug into their home and their old pack for time shifting fixed electricity equipment (home storage) -- this would skip the shipping of their old pack back to the gigafactory, and instead have it shipped to their home (usually a lot closer to where the battery swap is executed than the gigafactory), and there wouldn't have to be a gigafactory recycling step, and there wouldn't have to be the contention issue of selling a used pack to a new buyer; you'd be stuck with your own used pack, and you'd probably be pretty happy with it. See wk057's project, although, a Tesla conversion kit would work differently.
I could see a roaming swap truck service, that would pull up in an electric truck loaded with half a dozen to a dozen new packs to your home, do the swap right there by a specialist, and the same team would have an electrician installing your conversion kit into your home electrical system, and then they mount up the old pack into your home, and give you back your car and your house and drive off, one pack lighter on the truck.