Can you switch from a Non OA visa to a Non O visa after the first year in Thailand?

Possible using an agent.

The only way to get off a Non-OA is to wait until it expires. So either leave Thailand on the last day its valid and return to Thailand the following day on a visa exempt entry and go and apply for a Non-O at your local immigration office by meeting the requirements. If you are on the 2nd year you can get out of that visa leave anytime during that 2nd year without a re-entry permit and come back visa exempt as above. Or while out of Thailand you can apply for the Non-O at the Thai consulate that serves whatever country you are in

You can never change a Non O-A to a Non O visa, but you’ll get a two years stay on a Non O-A.

After two years you can apply for an Extension of Stay on the same condition as for the regular Non O, but you maintain the mandatory Thai health insurance requirement as well.

You can’t. With a Non OA you can get almost 2 years from the original visa if you enter just before it finishes and you have medical insurance.

After that you’d have to leave and enter on an exempt or tourist visa and convert to a Non O (or enter on a new Non O).

You can’t switch over in country.