Can I change my Non-B work visa to a Non-O marriage visa without leaving Thailand?

Not sure if this helps, but my husband did the opposite - changed his OA retirement extension to a work visa after he was offered a paying job. He had to cancel the OA, which was still in effect, and leave Thailand, & then apply for the work visa in his home country. Then entered Thailand with that new work visa. It required two steps, cancel the old, then get the new.

You cannot change a visa from one type to another. So are you actually on a VISA? Or are you actually on an EXTENSION? If you are on the original 90-day non-B visa, then you can't do anything except leave Thailand and apply for the non-O visa after resetting your status.

If you are actually on a work extension (the stamp in your passport says "extension of stay") then it might be possible, but that completely depends on your immigration office.

If you are switching because you will stop working, you would need to get documentation from your job that states your last day of work. Take that to cancel your work permit, and then take it to your immigration office and they will cancel your extension effective the date on the letter from your job. At the same time you can then apply for a new extension based on marriage to a Thai. You would need to be married at least 2-3 months depending on the office, and would need to be able to show the 400,000 baht in your account for 2 full months already at the time you apply.

If you are switching because you want to, and not because you will stop working, then you need to wait until the end of your existing extension. When you go to immigration for your next extension, you would simply apply for the extension based on marriage to Thai rather than work. You would need to meet all requirements for the marriage extension.