What is the process for changing from visa exempt to a non-O (retirement) visa in Chiang Mai?

My experience of applying to change from visa exempt to non-O (retirement). Thanks to Todd and others I completed the first step of my application for a non-O visa. For anybody that is new to the process at Chiang Mai immigration I'll share my experience yesterday (22nd July).

Go through the COVID screening station and get a "screened" sticker. If you don't know which forms you need go to the information window inside the main immigration building.

Once you have the correct forms and any other documentation, go to the seating area outside the building covered with a canopy. An immigration officer will check your paperwork, if it's in order you'll get a card with number. Go back to the information window inside the main building and give card. Your papers will be checked again and then you get a queue ticket. Wait for your number to be called.

I had a glitch of my own making, I was called to the window to change my visa exception stamp to a non-O visa (retirement). The immigration officer told me that I needed to extend the visa exception for 30 days, since it expired less that than the 21-days required. She told me that if I got the extension, they could process my non-O application on the spot.

I now have my receipt to go back and pick-up the 90-day non-O visa on 17th August.

The Chiang Mai Immigration staff were very helpful. Thanks again to Todd and others for their great advice. I've attached the requirements I received from Chiang Mai Immigration in case it's useful to others.