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.