How do I transfer my Thai Ed visa from an old passport to a new one?

I know you're in Chiang Mai BUT here's what you need to get stamps transferred in Bangkok. (so you might need these copies up there)

My partner did his new Passport in Bangkok (Swiss Embassy). For the stamptransfer att immigration he needed

1. New and old passport

2. Letter from the Embassy

Then it was “easy-peasy“. Worked very well. Is a standard procedure for the people at immi.

As George Marinkovich says. In case of you have extension of stay, and if your new passport was issued in Thailand, you need to bring the letter from your consulate you had received along with your new passport, if your new passport was issued in your country and you already used it to enter Thailand, you don't need the letter from your consulate.

If you really have an ED VISA rather than an "extension of stay" for purpose of education then you can't transfer the visa from your old to your new passport. You just need to use both passports, the new for everything except for your visa, the old only to show your visa.

If you have an extension of stay stamp you can get your local immigration office to do the transfer from your old to your new passport. They will probably want to see the letter you get from your country's consulate and the service should be done for free.