Well, there IS a way to do this, if you put in the effort. I have discussed this with the plate sellers at the swap meets, they know no other way. You procure black plates (a pair). You must make sure said plates are no longer in the Calif. DMV system (AAA auto club can verify this if you ask nice). You then go to the DMV website and research this plate number, testing it as a personalized plate. If it passes these tests, you go to DMV(make an appointment online), going down there with the DMV personalized plate form completed, giving some sensible sounding explanation for the meaningless set of numbers and letters your black plate has. If they bless it, you take the stickers they issue and stick them to your black plates.
Oh, one more piece of advise-order the personalized plate by mail and have it put on hold at the local dmv. That way you've removed the face to face debate with the local DMV agent, who may or may not be on a power trip. You just go to the DMV at your appointed time and have the plate assigned to your vehicle.
Don't ask me how I know.
All the best,