Can I convert a marriage visa to a retirement visa in Thailand?

You change the visa application at your next extension date. You need 800k baht for retirement vs 400k for marriage.

Marriage extension is good up until court signs of a divorce or at the end of the extension if the partner dies during the extension period.

We are.doing our first extension based on marriage ourselfs and its time consuming. Asked an agent and the look of fear on her face and suggested retirement visa which she could do for 25k baht. We got a 60 day extension added while we sort it out from Chon Buri Immigration as Petchaburi Immigration after what must have been 10 checklist visits still said no.

Trouble with wife?

Yes, you can apply for your next extension based on retirement provided you meet the requirements. This means being over 50 years old and either documenting twelve consecutive months with a minimum income of 65K baht per month from abroad, or having 800K baht in a Thai bank account at least two months prior to submitting your application.

No visa can ever be "renewed."

You can apply for extensions, which are brand new extensions each year, nothing is renewed.

If you have a marriage VISA, the 90-day non-O visa, then you can only apply for extension based on marriage. The extension must match the reason for the visa.

The following year, you can apply for an extension based on a different reason, since you are not applying to extend a visa, you are applying to extend based off a previous extension.