How can I change my 60-day tourist visa to a Non-O visa in Thailand without proving finances?

okay, you could indeed go from a tourist visa to a 90 day Non-O and then to yearly extension of stay at the Chonburi immigration office without having to bank 400K baht.

Seeing as you're a brit it would take you applying by mail to the UK consulate here in Bangkok and getting a notarized paper from them stating you get over 40K baht a month in pension or income. Unfortunately for you, the brit consulate DOES require people submit proof of their income before they write the letter.

As

@St***

, said once you apply for the Non-O at the immigration office it would go under consideration for 2 weeks and then you'd go back and get it inked into your passport along with a new 90 day stamp. Then when there are 30 days or less left on that stamp you'd go back to the immigration office (with another letter of income from your consulate) and apply for a yearly extension of stay

No mate. I went a month ago. I'm actually surprised here why no one told you already in this group about just going to savannakhet in Laos to get that visa once your tourist visa near the end.

Just get to Thailand then go savannakhet in Laos for the none o multiple entry visa based on marriage with Thai then come back into Thailand with a 15 months visa

You cant do it in Thailand without proving finances. It goes under 'consideration' for 2 weeks after you meet the requirements