You need two accounts & it only works this way:
Keep the current car and then buy the new one so you
own both at the same time (if you sell the current car before the new car is on your existing account Tesla will delete any remanning supercharger miles)
- Original account with current car and new car on it at the same time (SuC miles still registered against current car)
- Create a new account with a different email address
- Transfer via the app ('Sell') the existing car from the current account to the new account, leaving only the new car on the original account
- At that point, as soon as the original car is removed from that account the SuC miles stay and can be used by the new car
- The original car on the new account no longer has free miles and any SuC sessions will be charged to your registered card.
- Now you can physically hand back or sell the original car
However, I did this with two Teslas both purchased for cash and fully owned by me - it may not work if a company car is involved, only Tesla can tell you that. I made my transaction sitting in the Tesla Service Centre with their advisor on the phone and online to Tesla Netherlands as nobody seemed to have done this previously in order to retain the free miles and they weren't sure it would even work (it did)