Don't remember if it was Musk and exactly what was said and how it was said but at the Autonomy event it was stated that all cars with FSD would receive the HW3 upgrade and anyone upgrading to FSD would also get it. Also the it was stated that current Autopilot was at 80% usage of 2.5 all all future features would require FSD. All of this is of the off cuff paraphrasing but you don't need to be on the development team to watch the event.
Also to update all Tesla 2.5HW cars (S,X & 3s) to HW3 would be prohibitively expensive. This is part of why FSD cost so much is to cover the upgraded hardware.