Due to possible time constraints, I may have to get my new US passport done in the US and not here in Bangkok, unfortunately.  I asked the the embassy in Bangkok about getting visa/stamp transferred into my new passport, if I were to make new passport in the US.  They told me that if I do this in the US (and not here), once I have the new passport, I could contact them (in Bangkok) and they would send me the required letter (to take to immigration to get the transfer completed).   Now, obviously I can only "take to immigration" if I am in Thailand, so does this mean I can take this new passport, the letter and old passport and arrive back at BKK airport and handle it at immigration there once I land?  They wrote that if I need more information regarding this, to contact my nearest immigration.  Since my local immigration offices in Samui and Surat Thani do not answer phone calls (and I am on another island), not so simple -  hence this post.  

If you have a valid visa or a re-entry permit in your old passport you will show both passport when you check in for your flight and when clearing immigration when you arrive. Immigration will stamp into the country in your new passport.

Then you would have your stamps transferred at Samui immigration if that is your local office.

You will probably have to upload copies of both your passports to apply for a certificate of entry (COE).

1 hour ago, ubonjoe said:

If you have a valid visa or a re-entry permit in your old passport you will show both passport when you check in for your flight and when clearing immigration when you arrive. Immigration will stamp into the country in your new passport.

Then you would have your stamps transferred at Samui immigration if that is your local office.

You will probably have to upload copies of both your passports to apply for a certificate of entry (COE).

Thanks - this is great news.  My O-A is valid to end of Oct (plan to return early Oct for my last 90 days).  BTW, If I were to choose to convert to retirement visa upon my return here, would that option would still be open to me in light of my passport change?  

13 minutes ago, songmaew said:

Thanks - this is great news.  My O-A is valid to end of Oct (plan to return early Oct for my last 90 days).  BTW, If I were to choose to convert to retirement visa upon my return here, would that option would still be open to me in light of my passport change?  

A valid OA visa issued by an embassy or consulate allows a one year entry up to the day it expires.

You could choose to use your new passport to enter visa exempt and then change to a non-o visa at immigration. If you did that you would not need your old passport at immigration.