I have 2 questions that I will try and condense into 1. I am renewing my Australian passport in Australia. My old Australian passport has a retirement visa expiry date of 29-3-2015. When I re-enter Thailand at Don Muang in March will I be allowed in by showing my old passport with the stamp to 29-3-2015 or will they just give me a 30 day entry pass on my new passport? I also have an unused British passport. If I use that will they give me a 30 day entry pass in that? When I re-new my retirement visa in Chiang Mai will they change it to my British passport or are they loathe to do this? Any answers and information would be appreciated, thanks

You will need to show both Australian passports when you enter the country. I assume you have a re-entry permit that you will show in your old passport to be stamped into the country to March 29th when your extension ends.

If you used your UK passport you would only could get a 30 day visa exempt entry but that would invalidate your extension of stay you have now.

Immigration will not allow you to change passports from one country to another when you apply for an extension. The only way you can do that is leave the country to get a single entry non-o visa and apply for an extension using that passport.

Thanks Ubon Joe, that answers my question well