The MoPH Official stated that although the Cabinet had approved a requirement for health insurance for O-A Visa applicants and renewal (?? this has been unanswered question since the announcement since there is no such thing as a renewal of an O-A Visa - the question being will it also apply to those that entered Thailand on an O-A Visa and apply for an extension of stay or continued renewal of such extension),
I have to disagree when you say there is no such thing as a renewal of an O-A visa because this is done all the time. I traveled to and from Thailand for years on an O-A visa which I renewed every year. I never requested or received an Extension of Stay until the time came for me to retire and live here full time.
Those who have O-A visa's, and those renewing their O-A visa's, are required to show proof of insurance beginning in July. Those who are currently retired on Extensions of Stay do not.
As far as your question: Will health insurance be required for those who enter Thailand on an O-A visa and request an Extension of Stay? My assumption would be yes, they will be required to have health insurance for the mere fact that they are holding an O-A visa. The line in the sand has been etched. Those with O-A must have it - those with EOS don't.
Once a person receives an Extension of Stay (EOS) for Retirement purposes, that EOS takes the place of the original visa. That EOS stamp you get in your passport is technically considered a type of visa. The fact that this is not clearly described in Thai visa literature has led to a lot of confusion.