This is my, not so complicated and not scary, experience about changing my visa from non-immigrant F to non-immigrant 0.
I am French and for the last 30 years I have been working in Thailand on a non-F visa (it is not a very common one I know). Since I retired in December 2024, I needed to change my status to a non-O visa. Living in Kamphaeng Phet province, I went to the the immigration office there: it is friendly, not busy, there are plenty of rooms in the parking lot. I have one hour driving, but it is largely compensated by the fact there is barely any queue (I never had to wait more than 5 minutes).
My first trip to immigration was to get information. I was informed that I had to leave the country, that there was no way to change the visa type while staying in Thailand.
I went to a 36 hours journey in Phnom Phen (because it was the cheapest flight and I had already been there and wanted to see how was the country after 10 years). I came back and got the exemption visa (? the stamp automatically granted to French nationals) on arrival at Don Muang.
I got a bank certificate about the 800K in my bank account and new ID photos. I had no lease for my place of residence because I had been staying there for 30 years and it was always on an amicable understanding. I had a letter from my landlord and a bunch of electricity bills.
On my second visit to immigration, I was said that I needed a lease contract, all my proof of address where not valid enough. That was a visit for nothing, it was my mistake for not following what is written on the web site.
I went back 3 days later with a proper lease contract, signed and with the proper stamp apposed. My documents were checked and approved. I was informed that I would get a phone call later on.
Within a couple of days, I got a call from an officer who came to visit the place I am renting, my landlord, a retired police officer happened to be there too, the immigration guys took a couple of picture, we exchanged some plaisanteries and they were gone.
3 weeks after my previous visit at the immigration office, at the date I was given, I went back, they already knew my face, had my documents ready and after some stamping in my passport I now have a 90 days non-o permit to stay. I will have to extend it in early April.
One mistake I made, beside the lease contract, was to not request the re-entry visa when they stamped my passport, but I don't plan to leave the country during the next 3 months.
One thing that certainly made the process more pleasant for the officers and for me, is that I can speak enough Thai to go through the process by myself, not having to rely on the approximate translation by a spouse.
I had a lot of apprehension about that visa process, I never did anything myself while I was working, giving my passport to the person in charge and getting it back 4 weeks later, I never had any contact with immigration. I had heard some very bad stories, but in my case it was a very smooth process. The immigration office of Kamphaeng Phet may have some exceptionally friendly officers, I cannot compare, but for me, the process was really painless.