Can I change from a Non-Immigrant OA visa to a Non-O visa in Thailand after the OA visa expires?

Have you already extended your OA for 12 months? If not get new insurance and re-enter before the Non-OA expires and get a 12 month single-entry stamp

If you happen to be in Thailand after the last “enter by” date shown on your Non Imm O-A visa you can leave without a reentry permit and re-enter visa exempt (provided your from a country that is approved for visa exempt entry) and apply for the Non O. You don’t have to wait until the end of your permission to stay date. Just check to be sure you are past the last “enter by” date shown on your O-A. You can apply for the Non O from within Thailand.

You cannot CHANGE an OA visa/extension to an O visa 😕

𝗪𝗼𝘂𝗹𝗱 𝗜 𝗻𝗲𝗲𝗱 𝘁𝗼 𝗹𝗲𝗮𝘃𝗲 𝘁𝗵𝗲 𝗰𝗼𝘂𝗻𝘁𝗿𝘆 𝘁𝗵𝗲𝗻 𝗱𝗼 𝘁𝗵𝗲 𝗮𝗹𝗹 𝗽𝗿𝗼𝗰𝗲𝘀𝘀 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗱 𝗳𝗼𝗿 𝗢 𝘃𝗶𝘀𝗮 ?

Yep..

You can get OFF the OA visa/extension, by exiting the country, come back in on a visa exempt entry and APPLY for a Non-O 90 day visa at your immigration office

OR

by going to a thai consulate and buying a new Non-O single entry 90 day visa..