What should I do about health insurance for retiring in Bangkok with pre-existing conditions?

And please, terminology is so important if you're willing to give advice.

Retirement Visa?

a Non Immigrant O Visa from a Thai Consulate OR

a Non Immigrant O Visa from your local Immigration Office OR

a Non Immigrant O-A Visa from the Embassy in your country of Residence OR.

an Extension of Stay from your local Immigration Office.

These 4 options are all based on being over 50 years of age and willing to sit out the rest of your life in Thailand,

I see requirements from all options above and they are all correct if you name all Retirement Visa, but all wrong if yo use another option

Thai Immigration does NOT REQUIRE health insurance to retire in Thailand.

Go to your local Thai Immigration office and pick up the list of requirements for the Non O and Extension of Stay for retirement of persons age 50+.

The O-A Visa for age 50+ applied for in your home country does require health insurance.

If you arrive into Thailand on a exempt visa on arrival, tourist visa or Non O visa, it may be converted in local Thai Immigration office for retirement.

Be prepared to meet the financial requirements in Thailand for retirement.

Good luck.

Come on a tourist visa or visa exemption and convert it into a 'Non O visa for the purpose of retirement' here in Thailand.

In this way you bypass the need for thai insurance (which is for the non OA visa only). I recommend you get insurance anyway.