Hmmmm. I always thought that Model X and S always came with lifetime free supercharging. My wifey's Model 3 gets charged when she uses the supercharger, but I have never had to pay for any supercharging, ever, over the life of three Model Ses. So, one would never need to "transfer" supercharging to another Model S or X because they come with it.
If they don't, I'd like to hear of it. My daughter is planning on buying a new X, and she'd be ticked if anyone told her she'd have to pay extra to charge on a trip. And she's a big girl. I wouldn't mess with her, myself, and I'm 6'4" and 220 lb. Last time a Tesla salesperson argued with her, he got shredded.