Can I switch from a Non Immigrant O visa extension to a marriage extension before my current extension ends?

Doubt they'd let you cancel this current extension and apply for a new one but that's between you and your immigration office so go on over there with your thai wife and ask

Most likely they will say do it next year which means you will have to meet the seasoning requirements of this current extension for the whole year you're on it until you switch to the marriage extension next year

What that means is you need to keep the 800K baht IN the account for 3 months after the extension you're on was granted and then you couldn't let the balance go below 400K baht the rest of the year.

Then you'd be fine to apply for the marriage extension next year