Can I convert my retirement visa to a marriage visa in Korat without leaving Thailand and still meet the 90-day reporting requirement?

You can apply the next time for a new Extension of Stay based on marriage to a Thai National. There are more requirements to meet, best is to visit the Immigration office for the hand out with requirements, This Extension of Stay goes under consideration for 1 month, during this month Immigration may visit your address to check you are living together. After this month you get the remaining 11 months stamped in your passport.