I am a USA/UK Dual citizen.I currently have my Immigrant O extension 

In my USA passport and wish to have  it in my UK passport. Do I have to

start again and how would I do this without going to the UK and where is

the best Local country to do this. I am married to a Thai for many years,

would this help in obtaining an O visa locally. I would prefer not to take

the marriage route but If I decide to do this can I drop the marriage approach

when I apply for the extension? Ubon Joe's input would be much appreciated.

You will have to start over again to use your UK passport for the extension application.

You could get a single entry non-o visa based upon marriage using your UK passport at a nearby embassy or consulate.

You would need to leave Thailand by air using your US passport and enter the other country using your UK passport when you do it.

After getting the non-o visa you could then apply for a extension of stay based upon retirement.