Is there a service in Thailand for transferring my visa to a new passport, and what are the costs and waiting times involved?

I had my non-B extension transferred at CW 2 weeks ago (Irish passport) I didn't have the letter from my embassy nor was I asked for it.

Okay, let's sort out WHEN you go to the immigration office to get your stamps transferred and when you don't.

IF you are here on a 30 day visa exempt, a tourist visa OR a valid multi-entry visa (from a thai consulate in another country) you do NOT need to go to the immigration office to transfer your stamps. You show BOTH passports when stamping out of the country.

IF you have an extension of stay in your passport from the immigration office you DO go to the immigration office to get the stamps transferred. They'll move your extension, your entry stamp, and any valid re-entry permits.

One more thing IF you have a valid multi-entry VISA in your passport (that you got from a thai consulate in another country) you carry BOTH passports when exiting/re-entering the country. The old one with the valid visa in it and the new one that they will stamp you out and in on until the visa in the old passport expires.

So when they transfer the visa what does that mean? They do something in the system or they stamp the new pport?

On the basement of Promenada, there's a few small rooms with glass doors to the left of the main office, that's where you need to go. They'll check your docs at the counter desk to the front. It's free and it took me 1 to 2 hrs max. You need to get the letter from your embassy first though. Good luck 😊

For the French passport, you need a special document from the Consulate and then go to immigration to transfer your visa to your new passport. British passport?

If you have a Visa, you just use both passports till the end of that visa.