Just to add to what bp said, since WMP is a reference player, if it detects differences to the original file that was added to its playlist, then there might be issues. Make the changes within LW before you add it to WMP's playlist. And also remember that if you move the file from its original location then wmp will no longer be able to play it unless you search for it (unless you deleted it of course.)

That's why I go for other players like iTunes which has its own music library so everytime you add a file it adds it to its own library. Then there's no stuffing around with changes of location or deletion from playlist addition location or id3 tag additions.