is it possible to change an o-a to an o at this time?

The only way to change from a Non-OA visa to a Non-O visa is to leave the country and apply for a non-o at a embassy or enter the country with a tourist visa or visa exempt entry and apply for a non-o visa at immigration.

If married to a Thai you could change the reason for your extension of stay for retirement to one based upon marriage.

OP if married you can obtain extensions to your non O-A based on marriage and avoid the insurance requirement.