What stamp will I receive when re-entering Thailand with a new passport while having an extension of stay in my old passport?

When you re-enter Thailand with a new passport that has an Extension of Stay and re-entry permit in your old passport, Immigration will issue a small rectangle entry stamp in your new passport. This stamp will indicate your arrival date and the date you are permitted to stay, which coincides with the expiration date of your extension/re-entry in the old passport. It will also be noted that your extension/re-entry is in the old passport. After re-entering, you must take both passports to the immigration office where you obtained the extension to have the relevant stamps transferred.