I have no way to verify that yet, although you are correct in that the policy shouldn't change for existing plans. So far I haven't had to pay.
The plot thickens.
It does not appear your service center person was "confused" or misunderstood anything. I now believe there has been communication from the mothership that the policy should be to charge even those of us with prepaid agreements that predate the change.
My car is currently in for its second annual service and Ludicrous upgrade. I spoke with my service advisor today, and when he was going over my bill, about to take my credit card info, he was going to charge $68 or so for putting swapping my all-season tires/wheels for the snow tires/wheels. (I have 8 wheels, so there was no mounting or unmounting of tires involved.) When I explained that I had the prepaid agreement that covered tire rotation and wheel alignment, he said that he would waive the fee this time, but in the future I'd have to pay for those things. We had a short discussion about it, but I realized that this decision was not made at the service center level, so correcting it will have to come from corporate.
I checked MyTesla, to see what the agreement said. Here's something I find really less than forthright: the agreement now in my account, showing a date of when I signed it back in November of 2014, has been changed. Clicking the link to that agreement, the CURRENT agreement, dated in 2016, with changed language is displayed. Again, this is from within my MyTesla account, while logged in. A screenshot, including the text in question circled and underlined, is below.
Luckily I am also able to include a screenshot, also with the relevant language circled in red, from the agreement as it was written at the time I purchased it. To get that agreement, though, I had to dig through my old email.
Tesla should be showing us the agreements we entered into, not modified agreements when we try to see our own documents in MyTesla.
More importantly, Tesla should not be trying to change materially an agreement like this after the fact.
Below are the two screen captures, with the original first.
Original:
Current: