Most likely it's an upgrade to HW3 or Tesla calls it "Full Self-Driving Computer".
Thanks. That cleared up a lot.
Usually, if your car already has HW3 built-in (not upgraded) that means it also comes in with MCU2 built-in and there's no need for an MCU upgrade.
Since you need an MCU upgrade, that means your car did not come with HW3 built-in and it needs to be upgraded to that hardware level.
It's true that a car could have been paid for FSD as far back as 10/2016 but unless your hardware is HW3 and MCU2, you might want to upgrade them to the current hardware level.
Note:
MCU: Media Control Unit. It's behind the 17" central monitor.
HW3 or "Full Self-Driving Computer": It's behind your glove box compartment.
Those are 2 separate pieces of equipment.