I hope you are joking. No, the format will stay the same, WEBP VP8. Your Windows is fool-proof enough to detect correct format even when extension is wrong, but it won’t change the binary content.
Also, any conversion decreases the quality. So yes, we can convert to PNG, we can even upscale… But then what’s the point of paying to OpenAI, isn’t it better to just stick to Stable Diffusion or Midjourney?