The fact is that it is actually illegal, strictly speaking. If there is a visa crackdown in Thailand, your NON-O visa can be cancelled.
If you want a NON-O visa instead of NON-OA visa, you need to apply for a new visa at the Thai Consulate in another country, it cannot be just converted without leaving Thailand. This is a violation of Thai Immigration law.
Having said this, I know it has been done and is still being done for $$$ fee.
After all, This Is Thailand! 555