Car has been written off, CAT B. What now?

Hi, I have been told today my car is a CAT B write-off. I did not accept the first valuation, and I haven't heard back from them since.

Now that it's a CAT B, can I buy a car tomorrow and change the insurance from my previous car to a new one?

Is that okay to do this, I would assume it only matters that the car was insured during the tike of the crash.