M4a files seem to be changed when I installed new Windows Media Player...?

I had both M4a and MP3 files on computer...installed new WinMediaPlayer, I think the new WMP converted all my m4a files to mp3...anyone else have this happen...I tried to search in windows using the search by file type and the m4a file extension does not show up. I was thinking of converting my m4a files anyways...if you have any insight that would be great.