Can I switch from a Non-B visa to a Non-O marriage visa in Chiang Mai without leaving Thailand?

Just to add, you need to cancel your work permit first, and take that paperwork to immigration to cancel your current extension.

As you correctly stated you're not on a Non-B visa now you're on an extension from it, and you wouldn't be applying for a Non-O visa based on marriage you'd just be changing the reason for your extension from one based on employment to one based on marriage to a thai

You can do it by cancelling the extension you're on based on employment with documentation from your employer saying you stop work on xxx date,

AND

You get all the documentation you need for the marriage extension (400K baht banked in a thai account for 2 months before you apply, all the copies, marriage documents, house book thai i/d etc bank letter, bank statement, photos, etc)

You go to the immigration office (with your thai wife as she's the reason you can get that extension) you cancel the extension based on employment, and you immediately apply for a new yearly extension based on marriage to a thai.

You'll pay 1900baht, they'll give you a 30 day under consideration stamp and then when that runs down you go back to get the year extension inked into your passport.

All you're doing is changing the reason for your extension not changing visa types.

OR

You could cancel your extension, then bounce out/back to get a 60 day free entry stamp and apply for the in country 90 day Non-O visa first for 2000baht at the immigration office then when that runs down you apply for the yearly extension.

1 - yes it is possible to switch reasons for your extension

2 - no, you wouldn't come back on a tourist visa, you'd come back free stamp entry on a 60 day visa exempt and then apply in country for the 90 day Non-O, then the year after that

3 - you need a TM30, lease, land lord i/d house book, etc

GO to the chiang mai immigration office and get the hand out listing the requirements for a year extension based on marriage to a thai.

This is a general list of what can be required it is NOT office specific so that's why I said go to the C/M office and get their hand out