Not sure where you heard that a new harness is required.so the whole new harness required is irrelevant. Just install new headlights and beg tesla to set the "matrix headlight" setting to "on" on your car?
As I understand it if you have a US car made before global lights were introduced, and you want to retrofit taillights with discreet amber indicators/blinkers, and twin reverse and foglights, then you do need to do some custom wiring. European cars don’t need to because they always had one of each on either side. Everyone who fits global taillights on older cars has to use either 2 (Euro) or 4 (US) 3-pin to 4-pin adapter cables. Maybe that’s where you heard about a harness?
As far as the headlights go, they are a straight swap physically and electrically. Only software (gateway configuration) changes required, as @terranx said.
Tesla almost certainly won’t entertain changing the headlights type for you, as it’s not an official retrofit. If you know a senior tech there, then maybe. The people doing this retrofit are using people who “have the means” to do the coding, remotely, using a physically connected laptop.
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