Can I change my non-immigrant B visa to a tourist visa at the immigration bureau in Thailand?

𝗖𝗮𝗻 𝗜 𝗱𝗼 𝘁𝗵𝗶𝘀 𝗮𝘁 𝘁𝗵𝗲 𝗶𝗺𝗺𝗶𝗴𝗿𝗮𝘁𝗶𝗼𝗻 𝗯𝘂𝗿𝗲𝗮𝘂

That would be NO, you can't 'shift to a tourist visa'.

There are two possibilities of you being on a "Non-B"

If you are on the initial 90 day stamp from a Non-B visa (as in you never got it extended at the immigration office) you can stay the 90 days and then you have to exit the country.

Depending on your nationality you can re-enter on a free entry stamp OR you can go to a thai consulate in a nearby country and BUY a tourist visa before you come back

Now if you already got an extension from that original 90 day Non-B stamp you have to go to the immigration office with documentation from your employer and cancel that extension officially,

THEN you can leave the country and either come back free entry stamp or on a tourist visa you got from a thai consulate.

Any way you look at it you have to exit/re-enter the country to stay longer 🙂