@Oswald ******
just so you're clear about what you're doing.
You're not converting ANYTHING.
You already have a Non-O visa, (single or multi entry makes no difference) what you will be doing is applying for a "yearly extension of stay" based on marriage at the immigration office for 1900baht by submitting the correct documentation; photos, copies, bank book, bank letter, etc.
You will apply for the year extension, pay the money and you will get a 30 day under consideration stamp, during that time the immigration office might send someone to do an in home visit (or they might not), and at the end of that stamp you go back and get the year extension inked in. THEN if you're going to leave the country you need to buy a re-entry permit to go with that year extension.