Situation:

• Australian passport nearing the end of its validity,
• double entry tourist visa all used up and did a couple of back to back trips after that via Ranong to get me through the christmas period (first time they gave me 30 days which was great - meant I only had to do the run twice and didn't need to travel on New Years Day).
• Organised a new passport - picked it up in Bangkok yesterday,
• going to Savannaket on Tuesday 28th looking for a new double entry tourist visa..

Went to Immigration in Huahin today to transfer the stamp and departure card to the new passport and was told that this wasn't necessary as I was only on an entry permit, not a visa and to just hand both passports to the immigration departure office in Mukdahan when I leave and then use the new one when I come back.

Sounds good and easy. Any guidance on whether it is true or whether I am likely to get hit for 12 days overstay because the start date of the new passport is 16th January and technically the old passport with the duration of stay stamp in it became invalid on that day?

Should I go back to immigration and insist that they transfer the stamp?

What they told you is correct.

Just show both passports to immigration when you leave. They will put your entry/permit to stay stamp in your old passport in new one and stamp you out of the country in it,

No overstay because your original permit to stay date in your old passport is still valid.

many thanks to you Joe, it is always reassuring when someone who knows what they are talking about backs up what a random government lackey says