Can I switch my Non-O visa from a marriage extension to a retirement extension in Thailand?

There is no requirement that you need any health insurance to get a yearly extension of stay based on retirement from a Non-O visa..

You wanna switch the reason, meet the requirements and on the next extension apply for one based on retirement not marriage..

Now that switching back if you were originally on a marriage extension, switched to a retirement one and want to go back to marriage extension for what ever reason MIGHT be a tough row to hoe. Few immigration offices will let you switch back and forth, SOME make a point of telling you they'll let you switch the reason once, and if you wanna switch again you're going to exit/re-enter and start everything over from square one

CHECK with your immigration office well before you're going to switch reasons for your next extension to get the hand out with the correct documentation for the reason you're getting the extension