Can I switch from an ED visa to a marriage visa while in Thailand?

Can you go from a 90 day ED VISA to a Marriage Visa inside the country? NO you cannot

BUT

If you're on an "extension of stay" based on education (READ your stamp) you can switch the reason to an extension based on marriage

you get a letter from your school stating you stop studying (usually the date your current extension expires) you get all the documentation to apply for the marriage extension, you go to the immigration office, cancel the ED extension and apply for the new extension right then on the spot. You're not getting ANY visa at all. All you're doing is changing the reason for your extension from education to marriage