Can I use my guardian visa in an old passport upon returning to Thailand and when should I transfer it to my new passport?

Solid advice. I used both passports to leave Thailand, but, I had to go to immigration in Mukdahan, I live in Korat, I was told they can do it at the border, but with the amount of Laotian workers queuing for their visas, the border immigration officials suggested I pay for a taxi & it'll be transferred immediately, only took an extra hour and it was free.

I was thankful that the Thai immigration officer suggested I go to immigration in Mukdahan to transfer my current visa to my new passport. A taxi fare from the border to immigration is a pittance compared to the time it takes them to do it at the border crossing. All in all, very easy to do.

𝗪𝗵𝗲𝗻 𝗜 𝗿𝗲𝘁𝘂𝗿𝗻 𝘁𝗼 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱, 𝗰𝗮𝗻 𝗜 𝘀𝗵𝗼𝘄 𝗺𝘆 𝗴𝘂𝗮𝗿𝗱𝗶𝗮𝗻 𝘃𝗶𝘀𝗮 𝗶𝗻 𝗺𝘆 𝗼𝗹𝗱 𝗽𝗮𝘀𝘀𝗽𝗼𝗿𝘁 𝘁𝗼 𝘁𝗵𝗲 𝗜𝗺𝗺𝗶𝗴𝗿𝗮𝘁𝗶𝗼𝗻 𝗼𝗳𝗳𝗶𝗰𝗲𝗿 𝗮𝘁 𝘁𝗵𝗲 𝗕𝗮𝗻𝗴𝗸𝗼𝗸 𝗮𝗶𝗿𝗽𝗼𝗿𝘁?

You show both passports when you enter the country and they will stamp you in on the new one

𝘁𝗼 𝘁𝗿𝗮𝗻𝘀𝗳𝗲𝗿 𝗺𝘆 𝘃𝗶𝘀𝗮 𝘁𝗼 𝗺𝘆 𝗻𝗲𝘄 𝗽𝗮𝘀𝘀𝗽𝗼𝗿𝘁, 𝗰𝗮𝗻 𝗜 𝗱𝗼 𝘁𝗵𝗶𝘀 𝘄𝗵𝗲𝗻 𝗜 𝗲𝘅𝘁𝗲𝗻𝗱 𝗺𝘆 𝗴𝘂𝗮𝗿𝗱𝗶𝗮𝗻 𝘃𝗶𝘀𝗮 (𝗮𝗿𝗼𝘂𝗻𝗱 𝗦𝗲𝗽𝘁𝗲𝗺𝗯𝗲𝗿 𝟮𝟬𝟮𝟰) 𝗼𝗿 𝘀𝗵𝗼𝘂𝗹𝗱 𝗯𝗲 𝗶𝗺𝗺𝗲𝗱𝗶𝗮𝘁𝗲𝗹𝘆?

You go to your immigration office once you get back to where you stay in thailand and transfer the stamps over, you cannot wait until your next extension application in Sept 2024 to do it