Can I change my Non-O retirement visa to Non-O married visa before it expires in Thailand?

Most if not all embassies have stopped selling the one year multiple entry non-O visa. I would definitely check with the embassy to see if they are selling it, it's likely they just haven't updated their web page. You could also log into the E-visa application website and do a dummy application and see if it's an available choice

After you've taken your first 12 month extension based on retirement you, can when you apply for a new extension, change the reason to based on marriage by meeting the requirements