Can I transfer my visa exempt stamp to a Non-O visa based on marriage in Thailand?

There were never "old rules" that you had to get the initial 90 day Non-O visa outside the country. 😕 At the offices that can issue in country Non-Imm visas they've been doing them for years and years

Not every office can issue in country visas though so you need to check with your office.

Also you need to have a minimum number of days left on your current visa exempt entry stamp; at most offices it's a minimum of 15 days remaining, (at Chiang Mai it's 21 days remaining),

AND

you need to meet the proof of funds (400K baht in a thai bank account in your name only) along with providing the passport copies, bank letter, marriage documents, wife's house book, thai i/d, photos, TM30, lease, etc.

A 90 day Non-O visa cost 2000baht, you apply for it, they give you a receipt that says when you should come back to get it inked in, you show up on that day, they ink in the Non-O visa and a new 90 day stamp. THEN you wait until you have 30 days or less left on that and apply for a yearly extension (with all new documents, copies, photos, bank letters, etc)

What immigration office do you use (where are you in thailand)?