We had the same problem when we left in 2005 with four vehicles. We received new drivers license’ within two weeks of leaving and turned in the old ones to the DMV in SC. We notified them that we had moved and registered all four vehicles in SC but each time one of them would come due in CA they would send a renewal notice so we would notify them again but they kept sending delinquency notices. After repeated letters and phone calls, I finally got someone with some sense from the DMV on the phone and the notices stopped. It took over a year to get it straightened out.
What was really interesting was that it took us less than 30 minutes to get new drivers license’, and register four vehicles……….with NO appointment. We just walked in and we live in Greenville, SC which has 500k people in the county. Quite a contrast from the CA DMV.