MP3 file serial number?

I am not aware of any function in iTunes to simply "convert" bit rates. You can convert between file types, but this is a separate file creation process to make iPod or iPhone versions (only for formats that won't play on those devices), iPad or Apple TV versions (for video), or Creating an Apple Lossless version (which is only really effective if you are starting with something like a wav or aiff file, since mp3 or lossy m4a files would have no benefit.

You haven't shared this info here. I have to guess that if you used one of these conversion options, that you probably started out with something like wav? Those tags are not "embedded" when using iTunes for tagging - they are only kept in your iTunes library file. So some or all of your info will be lost when you try to use these files in any other external program.