Can I switch from a Non-B work visa to a marriage visa while in Thailand?

@Nayr *******

you need to get a letter from your employer stating it's okay to cancel your extension based on employment and get one based on marriage. THEN you would go to the immigration office cancel that extension and immediately apply for the marriage extension by submitting the required documents and meeting the proof of funds, (400K baht in a thai bank account in your name only for 2 months before you apply) OR documents from the tax/revenue dept and from your employer you make more than 40K baht a month at your job.

GO to the immigration office with your thai wife an talk to them about changing the reason, get the hand out they provide that lists everything you need, compile that stuff THEN go do it