There is an option to keep the original file. Under options in the "Conversion" tab, uncheck the "Delete source files after conversion" box. Then you can convert all your music into mp3s (probably put them in some other folder) and add them to the mp3 player, and also still keep all your wma files. Otherwise, if you convert the wma files and replace them, you will have to re-add them to your Windows Media Player library.