How can I change from a Non-ED visa to a marriage visa in Thailand without leaving the country?

There are a few details missing from Tod's post. At ChaingWhattana immigration there is a specific section to change your visa status. Once you follow their directions and get this done you will get a 90 day type O extension that gives you time to get all your documents, and meet the financial requirements for your year long extension. You do not have to have this all done before you go to change your visa status. They will give you a list. To do this part the first time you simply need your Thai wife with you, and your marriage certificates to show them. Once you have gotten all your documents together, and meet the first financial requirement in this 90 day timeframe, you go back with your wife, present all your documents, and if they accept them, at that time you get a 30 waiting stamp, during which time they check everything out, and can come and visit where you live, to in fact see that you are married and living together. On the date you are required to return, on this 30 day waiting stamp, you will be given your 1 year extension. This is the procedure I went through when I changed from a type M visa to my current type O/Thai wife visa extension, and going from your type ED extension will be the same, at a cost of bt1900.

You can't change an Non-Immigrant Type ED VISA into another visa :/

BUT

if you are on an extension of stay based on education you can change the REASON for your extension from education to marriage.

You get documents from your school that states you want to cancel your ED visa extension,

you get the documents needed to apply for a year extension based on marriage to a thai (400K baht in a thai bank account in your name only for 2 months, and the required paperwork, photos etc)

you go to the immigration office cancel your ED extension and immediately apply for a new yearly extension based on marriage. You will get a 30 day under consideration stamp and then when that runs out you go back and get the year extension inked in.

ALL extensions cost the same amount (no matter the length of time they're issued for) at every immigration office 1900baht.