What should I do to ensure my extension stamp remains valid while mailing my passport to Bangkok for renewal?

If you are on a non-Immigrant OA extension of stay and need to mail your passport to Bangkok for renewal due to running out of pages, it's crucial to make copies of your recent entry stamp, your current extension stamp, and your passport's data page before sending it off. Once you receive your new passport, including a letter stating you obtained a new passport in Thailand, visit the immigration office to have your extension stamps transferred to your new passport.