There's two tiers-- Enhanced Autopilot (EAP), and FSD. To own FSD, you need to have already purchased EAP. However, you do not need to purchase FSD to have EAP.
Enhanced Autopilot is (in simple terms) lane keeping + adaptive cruise control. It's designed only to work on highways, and does not need the HW3 board nor will owners that only purchased this get the new hardware.
FSD owners will get the HW3 board so that Tesla can fulfill the promise of providing FSD that the owner originally purchased with their car (or after-the-fact). FSD will (in theory) handle self-driving in most cases, such as regular street driving with stop lights, stop signs, roundabouts, etc.