Hello,
Ditto horndude,
Changing the extension from mpg to mp3
doesn't change the format of the file. The physical structure of the file will still be mpeg.
It's whether the hardware (mp3 player for example) or the application (Windows Media Player for example) supports that 'physical file format'.
So, if you alter the extension from mpeg to mp3 manually without any physical file format conversion, and the hardware or application can play back the file, it means the hardware or app. supports the physical format 'mpeg'.