I’m not sure if it’s changed, but I did it myself back in the nineties and no appraiser was necessary. Just collected a few comps (10 actually), submitted them along with a letter stating what I thought the value of my home was and sent it to the assessor’s office.

They approved, it my tax basis went down and from that point it increased at the standard 2% annually provided for by Prop. 13 (which is what I also believe covers this process).

At some point, the assessor can kick it back up to the original value including the 2% annual increase, but not until the value actually suppports it. I sold before that, however, so I can’t really provide any further detail on that aspect.