I understand your point of view on the P85 to P100 conversion. I was hoping that someday I could have a P100+, but alas, just have to get a P100D+ instead.There's not many people with the knowhow to do these swaps correctly, as in all of the software and hardware changes needed to truly upgrade the vehicle.
I've done quite a few of these upgrades, and pretty much have it down to a good routine. There's a lot of "gotchas" we've run into over the years, and a lot of things that most folks thinking they can handle this have run into only to be stuck dead in the water part way through. We've fixed at least two upgrades botched by third parties, including cross-country vehicle shipping from west-coast states.
Long story short, unless whoever you're thinking of using to do an upgrade has definitely and successfully done the exact same upgrade before (like 60 to 90 or 75D to 100D, etc), I'd suggest not being the guinea pig because it's likely to result in some major headaches. There's a lot of caveats to do this right, and very few people understand it well enough to do so correctly. Like variations in pack type with cars with or without air suspension... done incorrectly, you'll end up with premature tire wear.
There's a specific company that I won't name who's come up every time I've talked about Tesla battery upgrades/repairs/etc... seems they're spreading based on a few popular YouTube videos on the topic that are actually filled with a lot of misinformation on the subject that seems to be geared towards luring people in for unnecessary "repairs" at huge profits for them. They have been mentioned here as well on this forum a bunch of times. I'll point out that one of the botched upgrades we took on for a customer was started by this company. So... yeah.
Anyway, we have customers doing upgrades all the time. We do one or two per month nowadays now that a lot more packs are coming out of warranty. I actually almost almost try to talk folks out of them, too, since to me the upgrades seem kind of silly when you could sell the car and buy something new with everything upgraded.... but we still get a lot of people wanting them regardless. The only upgrades that I won't do anymore are rear-wheel-drive upgrades to a 100 pack, because there's no standard firmware for this and it causes other headaches. Everything else is doable one way or another. For every upgrade we actually do, probably 10 people are turned off from the idea either from sticker shock or realizing a sell/trade path may be better.
We're definitely in a unique position to offer the most reasonably priced pack upgrades/replacements possible in most cases. A caveat to that is the original 60 packs... at this point most of these are well beyond our degradation threshold for module resale, so we have to price things accordingly. If your car gets 140 miles at 100% charge, we're not going to be able to recoup the costs from your core pack like we can with an 85 that gets 250 miles, so your upgrade cost would be higher.
For folks with what appears to be excessive degradation or other pack issue, I'm planning on making a device to collect pack diagnostic data, for people serious about an upgrade, which we can ship to them for a deposit so we can come up with a net cost for an upgrade that fits the specific cases. For the most part we can price things over the phone.
That said, I may have an upgrade deal coming up (100 kWh Q1'2020 ludicrous capable 1700A pack), if any dual motor folks are interested. Would personally like to see this go a 2014-2015 P85D owner.
I will also comment that the Supercharging speed increase of going from a current 85 pack to a 100 pack can be noteworthy for long trips. If you find your charging times have increased beyond acceptable range, but still want to keep the car, the 100 upgrade might be the way to go. I haven't looked into the Supercharging speed changes on a 90 pack???