The chargers do all the AC to DC work the V4 standard for NACS has 400-1200V certs with rates up to a megawatt. There is never any voltage or current on the cord or plug until it’s fully seated and the guard pin which cannot be touched makes contact with the guard sleeve. This can only mechanically occur when the plug is fully seated into the receptical. Then the CANBUS architecture is used to send coded data over the signal pins to establish a handshake. That handshake tells the charger what voltage and what current to send down the HVDC pins and also establishes billing information if its a Tesla. Then and only then will the charger send the requested voltage down the cable. Since Tesla SC are part of the NACS standard all of them will have the DC/DC at the V4 at the V4 standard it’s software to up the voltages in most of the chargers already installed and the very old ones will need a new DC/DC transformer stack with more turns to get to 800+V the revenue a SC generates is in the $100,000+ per year each stall so a thousand dollar transformer in the very old ones is just a statistical O&M expense and fully deductible at that.
I think it is unlikely Tesla would upgrade their older superchargers. At an 8 stall station, 2 stalls share power. It seems to me the distribution transformer would need to be upgraded as well to bring it line with a V4 station. Musk just fired most of the supercharging division, so there is no one around to to the work if feasible.