You wouldn't be changing ANY visa :/ You are most likely on a yearly extension of stay based on being over 50 (retirement) that you get from the immigration office every year for 1900baht.
You would be changing the reason for your extension. In Bangkok the way you would do it would be to go in, cancel your extension based on being over 50, and immediately apply for a new extension by supplying the required documentation.
I dunno about CHiang Mai but I know someone who does.
@Shayne *********
, can you weigh in on this thread? Can they go from a retirement extension to one based on volunteering in Chiang Mai or will they have to go out and get a new Non-O ?