The easiest way is to allow the customer to decide to transfer the FSD license to a new vehicle, or to sell the FSD license with the old vehicle.
The other way would be to sell it as a service instead. These days most SW companies seem to be going to the service route.
Another thing at play is Tesla made a promise to customers that the vehicles were capable of FSD, but they're not. So at some point they're going to have to fess up. One way to handle that would be to offer them the ability to transfer the FSD license to a new vehicle with HW4 (or some other HW version that is capable of FSD).
Building on the idea that your best customer is an existing customer Tesla should consider these points expanded from S4WRXTTCS and other's suggestions of how licensing FSD software may work (and its less capable subsets):
1. Make the license available to purchase even if you don't currently own a Tesla vehicle.
a. Consumers not ready to purchase a vehicle but interested in locking in FSD at the current price can buy now.
b. Each purchase would come with a profile at tesla.com to manage your Tesla account.
c. Tesla gets paid to build a marketing database listing consumers very likely to buy a vehicle in the future in addition to other Tesla products.
d. Software license sales add to Tesla's bottom line; unconstrained by vehicle supply.
e. Refines the business of FSD license sales for use in future third party vehicles.
2. FSD is licensed conditionally to the individual and the first vehicle they apply the license to. Transfers to future Tesla vehicles or new owners upon the sale of the original vehicle may be done with a license transfer fee, i.e. 10% of the current FSD price.
a. Maintains promise of getting the full capability of FSD with consideration for what was paid into the program.
b. Current Tesla vehicle owners are less apprehensive to upgrade to new Tesla models like they are now without the ability to transfer FSD.
c. The decision to bring FSD vehicle trade-ins to Tesla loses some negative stigma associated with not getting good value for purchased FSD.
d. Tesla gets paid on the private and third party dealership transfer of Tesla vehicles every time new owners want an FSD feature.
3. Consumers with multiple Tesla vehicles would still pay for multiple licenses if they want FSD on more than one vehicle.
a. Continues a clean business plan of one license/one vehicle for development cost/risk.
b. They can afford it.