How can I switch from a Non-B visa to a Non-O visa in Thailand without leaving the country?

You can switch 'extentions of stay' just by meeting the requirements, for an extention based on marriage the main requirement is 400,000bt in a Thai bank or 40,000bt per month income

While you can't change a non-B VISA to a non-O VISA inside the country you CAN change the REASON for an extension of stay from one based on employment to one based on marriage to a thai :)

SO you can do it.

You would take a letter from your employer stating they have no problem with you canceling your extension based on employment and applying for one based on marriage

You would meet ALL the requirements to get a yearly extension based on marriage

You would GO to the immigration office and cancel your extension based on employment and immediately apply for a NEW yearly extension based on marriage to a thai.

The immigration office will cancel your current extension and give you a new "under consideration stamp" for 30 days based on marriage, THEN when that runs out you go back to the immigration office and get the year extension inked into your passport.

I think what you really are on is an "extension of stay" from the immigration office

how about you look IN your passport at your current stamp and see if directly under the location of the office it says "extension of stay permitted until" and has a date on it of November?

You can't be on a Non-B that will expire in november :O