Yeah, it's a huge undertaking. But in principle, there's no reason they can't add additional transformers to handle the extra capacity required, then convert it a piece at a time.
For example, if they have a supercharger on 3-phase 480V service at 150 kW per pair, at 312.5-ish amps per pair, they need roughly 1875A, so it probably has at least 2000A service (6 pairs * 300 amps).
For a V3 supercharger, they would need 250 kW per station, which turns out to be about 520 amps per station. So they could use the existing service to feed at least three V3 supercharger stations (and maybe four, depending on how accurate that 2000A guess is). If they wire up additional transformers, they could convert most of the stations a few at a time and add them to the new service, then wire up the last few stations to the old service.
What you're suggesting is hugely expensive with limited reward. V3 only helps existing long range Model 3 vehicles, and then, only below about 40% state of charge. Once you're into the taper, it's no faster than a 150 kW V2 station, despite investing large amounts of money into upgrading the electrical service.
V3 is really best on interstate travel routes where you're looking to "splash and go." In cities and suburban areas, people are looking for higher states of charge where V3 doesn't really help, the exception being travelers who are looking to "splash and go" as they travel through the city.
V3's high power output will only be fully-utilized by future vehicles with larger batteries like the pickup truck, roadster and semi. Possibly a future S/X refreshed with an improved battery cooling system (this can be done while retaining the 18650 cell format, if that's what Tesla wants to stick with, as it seems they do).
Finally, a little anecdote: my long trip charging stops are currently under 20 minutes on average with a long range Model 3 and 145 kW charge rate. There is only so much faster you can make a bio break before getting back on the road, so increasing charging speed is going to have diminishing returns on reducing my charging stop time. For the current fleet, the best value is more chargers, not faster ones. That said, not having to share power on V3 will help improve vehicle throughput.
Just my $0.02 on the matter.
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