Its just like any other insurance in CA so you can switch at any time. The main difference is, its pretty easy to sign up online (very easy in fact).
They had decent rates too, but a couple things stopped me from signing up with tesla insurance.
1. I like all my insurance products with one company (home, auto, umbrella) and I have both a model 3 and an ICE vehicle still, that would not be able to be insured with tesla insurance. this ment I would have had to insure the model 3 separate from everything else.
2. I am not "tinfoil hat" guy, but I am in IT, and tend to be a fairly private person. I dont have a facebook account for example (by choice), dont really do social media other than some forums, set up my video game consoles to always display as if I am offline, etc. I did not like the idea of the company with all of the telemetry data for my car (tesla) also being the one who insured my car.
I dont drive recklessly or anything, but I do enjoy driving the back roads home every once in a while, for example. Just like I would never ( ever ever ever ever ever) get one of those ODB devices insurance companies offer to possibly reduce your rates (because I think they would also use it to either raise your rates, or say you were at fault because you were going 8 miles over the local speed limit), I didnt want tesla having that information and being my insurer.
So, because I didnt want tesla as my insurer and also having every single datapoint about where my car had been, is going etc, it ruled out tesla insurance for me. I know tesla has all those datapoints anyway, but they are not my insurer, and have no duty to provide that information to anyone but me from an insurance standpoint.
Others dont care much about that stuff, and tesla's rates seemed very competitive, so definitely something to consider if price is a concern and you are in an area they serve.