Can I change my Non-B visa to Non-O based on marriage and how do I apply?

You were on the ideal visa extension, as your permission to stay was for 2 years 9 months through the BOI company you worked for.

And to add to Tod’s response, if you switch to the Non-O Marriage extension, you can keep this visa type. If you later get a job, you can obtain a work permit under the same visa, so there’s no need to switch back.

𝗧𝗵𝗲 𝗰𝗼𝗺𝗽𝗮𝗻𝘆 𝘀𝘁𝗮𝗳𝗳 𝘁𝗼𝗹𝗱 𝗺𝗲 𝘁𝗵𝗮𝘁 𝗜 𝗰𝗮𝗻 𝗰𝗵𝗮𝗻𝗴𝗲 𝘁𝗵𝗲 𝘃𝗶𝘀𝗮 𝗮𝗻𝗱 𝗲𝘅𝘁𝗲𝗻𝗱 𝘁𝗼 𝗡𝗼𝗻-𝗢 𝗯𝗮𝘀𝗲𝗱 𝗼𝗻 𝗺𝗮𝗿𝗿𝗶𝗮𝗴𝗲.

𝟭. 𝗜𝘀 𝘁𝗵𝗮𝘁 𝘁𝗿𝘂𝗲?

They told you wrong, you wouldn't change to ANY visa, all you would do is before the cancellation stamp runs out, GO to Chaengwattana and apply for the year extension of stay based on marriage to a thai (by meeting the financial and documentation requirements)

𝟮. 𝗗𝗼 𝗜 𝗱𝗼 𝘁𝗵𝗶𝘀 𝗮𝘁 𝗖𝗪 𝗼𝗿 𝗖𝗵𝗮𝗺𝗰𝗵𝘂𝗿𝗶 𝗦𝗾𝘂𝗮𝗿𝗲 (𝘄𝗵𝗲𝗿𝗲 𝘁𝗵𝗲 𝗡𝗼𝗻-𝗕 𝘄𝗮𝘀 𝗶𝘀𝘀𝘂𝗲𝗱)?

You're done using Cham Churi anymore. You cancelled your extension of stay based on employment (you weren't on any visa but on an extension) and now if you want to apply for an extension based on marriage you go to Chaengwattana main immigration office Section L-1

𝟯. 𝗖𝗮𝗻 𝗜 𝗷𝘂𝘀𝘁 𝗴𝗲𝘁 𝗮 𝘀𝗵𝗼𝗿𝘁 𝗲𝘅𝘁𝗲𝗻𝘀𝗶𝗼𝗻 𝗮𝘀 𝗜 𝗵𝗮𝘃𝗲 𝗮 𝗯𝘂𝘀𝗶𝗻𝗲𝘀𝘀 𝘁𝗿𝗶𝗽 𝗶𝗻 𝗔𝗽𝗿𝗶𝗹 𝗮𝗻𝗱 𝗰𝗮𝗻 𝘁𝗵𝗲𝗻 𝗷𝘂𝘀𝘁 𝗰𝗼𝗺𝗲 𝗯𝗮𝗰𝗸 𝗶𝗻 𝗮𝗻𝗱 𝗱𝗼 𝘁𝗵𝗲 𝗻𝗼𝗿𝗺𝗮𝗹 𝗲𝘅𝘁𝗲𝗻𝘀𝗶𝗼𝗻 𝗽𝗿𝗼𝗰𝗲𝘀𝘀 𝗯𝗮𝘀𝗲𝗱 𝗼𝗻 𝗺𝗮𝗿𝗿𝗶𝗮𝗴𝗲?

In theory you have until Mar 19th to apply for a new extension. I would say that you should get the documentation together, apply for the extension for 1900baht, get the 21 day under consideration stamp and THEN get a re-entry permit to exit/re-enter and not lose that stamp

Now can you get a 60 day visit thai family extension? I don't know as I've never heard of that being issued on a cancellation of extension stamp (the stamp that lets you be here now).

All you can do is go to Chaengwattana and see if you can get it, but again I'd say just get your ducks in a row documentation wise and go apply for the year extension FIRST, then travel..