@Andy *****
you can go from an extension of stay based on employment to an extension of stay based on retirement inside Thailand. There is nothing stopping a person on entering a non-b visa from getting an extension of stay based on retirement. After all, there is nothing written on the Non-B visa that says “retirement prohibited”.
However, it’s different for people going from an extension of stay based on retirement which they got from a non-o visa because they are over 50. On the Non-o visa they used, there are the words “employment prohibitive” written on the visa.
However, different immigration offices do have different policies. Hopefully that makes sense.