There is a lot of retirees come to Thailand meet the love of their life and before they meet them they holding a retirement visa. So if you are one of those retirees that want to get married with your Thai girlfriend you can change your retirement visa to marriage visa once your retirement visa need a renewal.
This is the requirements to apply for a marriage visa
Note : This option may not be applicable to all Immigration offices in Thailand. Requirements may then vary and list of documents needed may differ from every location.