What should you do with an old Thai visa when you have a new passport?

If you have a new passport but an old Thai visa in your old passport, take both to your local Thai Immigration office to request the transfer of your visa or extension of stay to the new passport. Ensure you have copies of necessary passport pages, visa forms, and possibly a letter from your embassy for the transfer. Experiences shared indicate that the process is generally straightforward, though requirements may vary slightly based on the immigration office.