sorry to disagree with you but was told by Immigration that my exemption added tourist visa could not be converted. I had (or wanted) to go out of the country and return with another tourist visa to start the process.
Unfortunately I am in the same position and so the only way to get a Non O is through “an agent”. We cannot do the mandatory out/in border run to get a 15 day tourist visa first. 800k Baht for 2 months in a bank and letter of confirmation from the bank. You would need 6 months of monthly payments to yr Thai bank account. Also insurance confirmation required according to my local immigration official + the normal residence, passport papers etc.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.