Can I convert a visa exempt entry to a Non-O retirement visa at Chiang Mai Immigration?

Remember Chiang Mai requires you have at least 21 days left on your current visa exempt entry stamp or an extension from it to apply for the in country Non-O visa because it goes under consideration for 20 days before you go back to get the Non-O visa and a new 90 day stamp inked into your passport.

SO

go to the main office by the airport, get the hand out AND if you don't have enough time left on your current entry stamp, apply for the regular 30 day extension for 1900baht too.

As Ellie said... Yes you can get the Non O for retirement followed by the one year extension of stay based on retirement at Chiang Mai immigration.

Suggest you ask for the list of requirements to do so at Chiang Mai immigration following your ASQ quarantine.

you will be able to apply new non-O visa based on being over 50 at the immigration office meeting all requirements unless your passport is from several countries that are prohibited in-country visa application (which I doubt yours is since you are in Thailand on visa-exempt entry).