Can I switch from a Non-ED visa extension to a marriage extension in Chiang Mai without leaving the country?

First you need to remember you're NOT on ANY visa at all, you're on an extension of stay

There is nothing that stops you from "changing the reason" for an extension and applying for a yearly extension of stay based on marriage from your ED extension.

You'd get the documentation needed for the marriage extension, meet the financial requirements, get the documents from your school, go to the immigration office that issued that extension, cancel it and then immediately apply for a new yearly extension based on marriage.

Take your thai wife and go talk to the immigration officers at the main office in Chiang Mai just to be sure it can be done, AND DON'T say you want a marriage VISA because they'll tell you that you have to leave the country to get a visa. All you want to do is to change the reason for your extension from education to marriage