What documents do I need to transfer my guardian visa from my old passport to my new passport in Thailand?

Follow the list that

@Se***

provided as that's what you need to do out at Chaengwattana to move your stamps to your new passport.

If you have a VISA like I mentioned you do nothing because the immigration office will NOT move a visa to a new passport. You travel with BOTH passports until the visa in the old one expires (although you'll get stamped in and out on the new passport).

However If you have an extension of stay, you take both passports along with the letter you got from your consulate when you got a new passport to the immigration office and they will move your last entry stamp, your extension and any valid re-entry permits. Depending on the immigration office you're going to you might need copies of the relevant pages in your old passport (or not).

Do you have a year-long, multi-entry Non-O visa that you got from a thai consulate in another country (where you leave every 90 days to get another 90 day stamp)?

OR

Do you have a year-long, extension of stay that you got from the thai immigration office inside the country?