can you trade in a model s you payed cash for on a new ms
You can certainly take advantage of the loaner trade in program. This is where the loaner will be discounted for $1 per mile driven or $1k for each month in the showroom (this portion my be discontinued).
Your MS would be valued in similar fashion.
If you were to trade in your current MS for a new MS reservation, then you'll have to call Tesla directly to see if you can work something out...