I've lived and worked as a teacher in Thailand for fifteen years. I've also been married to a Thai for fourteen of those years. Due to paperwork issues (lack of a BA), my job title was changed by the Labour Office and they were happy to issue me with a WP and the Immigration were happy to issue me with a visa based on this new job position. This went on for four years. Last year Immigration said that they wouldn't honour this WP, offered me a 'special' visa for 9000 baht!! This year I plan to change my visa to a marriage based one (I have the documents and the money in the bank) and then go to the Labour Office, with the school's documents and apply for a new WP. Does this make sense? Feedback appreciated.

You would change the reason for your extension of stay (a visa cannot be changed).

Others have done it.

Thanks for your reply. Yes I understand that I'm just changing the reason for my extension of stay. I'm just a bit nervous about changing my visa first (to a marriage based one and depriving someone of their tea money) and then going on to get my WP. I have a life here with my wife, daughter, teaching, farm etc.... The Visa/WP time of year always gets a bit stressful! Have a good festive season.

Do I need to leave the country to change change the reason for extension to marriage?

You will not need to leave the country. 

You will need to a letter from the school requesting your current extension be canceled and then apply for an extension based upon marriage,

Thank you.

Take care.