My experience switching from Non O-A visa to Non O extension at Phuket immigration.
When my admittance stamp based on O-A was finished I left Thailand and returned on a 90 day Non O visa issued in my home country. I collected the handout from immigrations in Phuket and followed that instruction step by step. I used the German consulate (Phuket) affidavit as financial proof.
I attach the latest handout dated September 2022.
All went smooth and I "sailed thru" without problems.