that time i did it in chiangmai because i already fly to lampang. the lampang immigration officer told me to go to chiangmai because only can do at international airport. i went to the international departure area and there is an office in there for fixing this type of problem. you can just ask any officer (you can ask the information or the vat refund officer) in bangkok airport where is the office located.
happened to me before, the immigration told me to go to airport because they don't have the authority to change it. you go to airport and tell the officer in there, not the one behind the security
