If you arrive at Bangkok Airport with a brand new passport (having got a new passport while back home), do you enter the new passport details on the TM6 card, even though your Extension of Stay stamp and Re-entry permit are in the old (cancelled) passport? I understand that both passports need to be presented to the IO, but confused about which number to use on the Arrival card.
TLDR : Answer Summary
When arriving in Bangkok with a new passport, it is advised to enter the details of the new passport on the TM6 arrival card, while also presenting both the new and old passports to the Immigration Officer (IO). The IO can stamp the new passport to link it with the visa and extension of stay in the old passport. It is essential to update your bank account details associated with the old passport number to avoid issues with international transactions.