I cant remember whether you can buy the pre-paid but I bet you can since that's just maintenance. I do know you can't buy the extended warranty on a CPO. It would be best to shoot an email to Tesla support to be sure if that is important to you before you decide.
Just to be clear, I'm not asking about the extended service plan. I'm asking about the prepaid annual service checkups. I'm still getting conflicting information about that from different service centers.
Basic Vehicle Limited Warranty
Subject to separate coverage for certain parts and the exclusions and limitations described in this Pre-Owned Vehicle Limited Warranty, the Basic Vehicle Limited Warranty covers the repair or replacement necessary to correct defects in the materials or workmanship of any parts manufactured or supplied by Tesla that occur under normal use for an incremental period of 4 years or 50,000 miles (80,000 km) from the first day and at the mileage a Pre-Owned Vehicle is delivered to the first Pre-Owned purchaser, whichever comes first, irrespective of the expiration date or mileage of the Basic Vehicle Limited Warranty specified in the original New Vehicle Limited Warranty for the Pre-Owned vehicle.
Answering the OP's question, I was just told that you cannot purchase prepaid service plans for a CPO Tesla. Here is the quote:
"Unfortunately, the prepaid service plans are only available to the original owner of the
vehicle, and not available to the subsequent owners, where purchased as a CPO vehicle, or through a private party."