A non-OA is a retirement visa. I'm assuming you mean change to a non-O (over 50) visa? In this case, you ensure the original visa has already expired, leave the country and either apply for the 90-day non-O at an embassy and re-enter on that, OR re-enter on a tourism visa/exempt and apply for the 90 day non-O at an immigration office
Flav Ien. I've lived in Thailand for 12 years. I have never once used a scummy agent. I do everything myself and save myself tens of thousands of baht. But..... It's each to their own! 😆😆😆