already stated, you are not changing your visa (you dont have one) you are changing the reason for your extension of stay. When you fill in the form TM.7 it asks you for reason for extension, simply change that to marriage, and provide the correct supporting documentation. I have done this (albeit the other way around) without any problem.
Well, I am the only one to my knowledge, until I ask about it and get a reply here. My question was can they change the central database at local immigration, or do i have to exit and enter the country again.