You could give anyone who bought FSD a credit if they want. When subscription starts they make a choice. Keep it forever on that car only. Or get a credit for months since bought at new subscription price.
So if you bought for $8,000 43 months ago (not sure what price was then) and subscription price is $100/month you have spent $4,300 and have 3,700 in credit against future subscription. If Tesla feels some of those early months was not worth full price they can award extra for those.
Again, there would be a choice. Everyone should be happy.
Letting all FSD owner until now transfer FSD forever would be an extremely costly thing to do. There must be a couple of hundred thousand who has bought now. If only 100,000 continue to drive Tesla for 20 years after they get their next Tesla that would be a loss of 100,000x12 monthsx20 yearsx100/month=$2,4 billion.
If it's 200,000 owners and $200/months then it's $9,6 billion.
I would be very upset as a stock owner if they give away billions.