Can I switch from a marriage visa to a DTV visa, and do I need to cancel the marriage visa first?

Yes, you can change from a marriage visa (Non-O) to a DTV visa, but you need to exit Thailand first to cancel your marriage visa and then apply for the DTV visa at a Thai embassy or consulate in another country. It is generally advised to obtain a re-entry permit if you plan to keep your current visa active while abroad; otherwise, leaving without it will result in the cancellation of your marriage visa. Users have reported that once approved for the DTV visa, the old visa is automatically canceled.