They need to do something to keep the 3 from cannibalizing S sales, especially as the backlog in 3 orders gets shorter. The 3's interior is almost the same as an S viewed from the side, but viewed from the front it's narrower. it's more efficient due to lower drag and lighter. A base model S 60 is slightly over twice the price of an equivalent range stock model 3. Very few people are going to pay double for a few inch wider car, extra trunk/frunk space, and a 2 inch bigger screen. And tesla has no reason not to offer all the high margin upgrades on the 3 that it can if it already developed them.
* Definitely change to 2170 packs as soon as they have the engineering done and cell/pack production exceeds the rate they can build 3's. Cheaper, lighter so better acceleration and range at same energy capacity, could potentially fit more energy in, but per musk's comments they don't see much demand for >100kWh after the lighter packs improve performance.
* They are very likely to start offering more premium upgrades to the S - the extra space can fit bigger, better seats, which is still a negative for some people comparing to mercedes, audi, volvo, lexus luxury. The director of interior engineering from volvo, the AR/HUD people they have hired from microsoft and others, have all been doing something for the past 1-2 years that we haven't seen yet.
* any tech they have on the 3 at the time. The last thing they want is a customer who likes the S but buys a 3 for one option not on the S yet. this may include HUD, the allegedly improved HVAC,