I have a 2018 RWD that I am looking to upgrade to the latest Model 3.
Can I transfer my upgrades to the new car?
BTW if you are planning on doing a trade in I'd wait a couple of months for battery day. They just announced a 395 mile range for the Performance on the Chinese rating cycle. Either the Chinese system is completely screwed up or they have put a 100KWh battery into the made in China Model 3s.
My understanding only (YMMV):
As bj mentioned - you can't move purchased options between cars.
But you can between owners -
If you buy a private sale Tesla, then the upgrades / features on that purchase do transfer to you (the new owner).
If you trade your car into Tesla then they can strip or add options as they see fit before making it available for purchase at the price they set.