Can I switch from a retirement visa to a marriage visa in the middle of the year in Thailand?

No, you have to wait until the end and apply to switch when you apply for an extension. You also need to keep the minimum amount of money in your account the entire year.

The only way to switch mid-year is to leave the country without a re-entry permit which will cancel your stay, and then you'll need to start over with applying for the 90 day non-O visa at an embassy outside of Thailand or applying to convert at immigration.