Unfortunately, based on the little bits of info I've gathered on this, I'm doubting Tesla will actually do the upgrades for free.
Changing their tune on that now, while it would likely mean that such a lawsuit could be avoided just because customers concerned about it would likely be appeased, probably won't happen for a few reasons:
a) They're still selling the Ludicrous mode changes as a $10,000 add-on for new P85Ds, meaning that if you don't choose that option you're still getting the power output that exists currently... which is totally unspecified so can be whatever they want really. So one way people would see this would be that they're giving existing owners a $10,000 upgrade for free.
b) More importantly, offering it for free to existing owners, especially if it is to fix their initial claims of 691 HP, would essentially be admitting their initial false advertising and thus likely making such a lawsuit easier for them to lose. While currently there are many owners who aren't even aware of this issue due to the impressive 0-40-ish straight line speed, many more would become aware of the initial issue if this were the case and thus many more chances for them to be sued. Keep in mind that the majority seem to be dismissive of this issue for whatever reason.
c) I'm not convinced the P90DL (or whatever we're calling it) or a Ludicrous-updated P85D is going to be capable of reaching the originally advertised 691 HP number on a dyno anyway, making such a giveaway to fix the original false advertising not really a fix anyway.
No matter how you slice it, providing the 691 HP number was false advertising. This isn't something the media pulled out of their butts, folks, like a lot of other numbers thrown around about Tesla. This was on the Tesla order page when I bought my P85D. While they may be able to get a pass on a technicality currently by not actually showing the combined number anymore for current buyers they certainly do not for anyone who purchased while the order page showed 691 HP. Combining the output numbers implies that the car could output this much power. Period. Since multiple tests confirm that it comes no where close, I'm pretty sure the implications of this are pretty black and white.
A free or at least reasonably priced (maybe $1-2k) update to make the car get reasonably close to the advertised 691 HP would be fine by me and would certainly put legal action out of my mind and the minds of many others, but at the same time it would likes put the thought in other's minds who hadn't previously considered it and who are previously unaware of this issue.
Currently no service center or person I've spoken with at Tesla even has any info whatsoever on the P85D Ludicrous mode retrofit nor any information on when it will be available for purchase... which I find odd since the CEO made a public announcement on the cost and all and many I spoke with weren't even aware of those details.
Tesla's lack of communication strikes once again.