What health insurance providers are recommended for renewing a Non OA Retirement Visa in Thailand?

You have to purchase a plan through the governments site. Bobby posted the link in the comment above me.

Here's a list of all the Thai companies offering the specific OA insurance. Have a wander through and get a few quotes

You can't change from OA to O within the country. My advice is stay on OA and get insurance whilst you still can. Once you've got it, it is renewable (certainly in Pacific Cross) up to age 99. There is a possibility the 0 for 50+ will be discontinued in the next two years. Already some immigration offices don't issue it, and others are following, so if you want to avoid getting married, getting a job, getting a degree, having a kid or continual border bouncing and visa runs, just get the insurance and have peace of mind in your golden years

Try Mr. Prakan broker. Very good service. I use Pacific Cross through them

Depends on Age.