In itunes select edit> preferences> import setting change "Import Using" to MP3 and use setting to select the quality slect ok and ok highlight the tracks you want as mp3 and righthand click and select create MP3 version......
OK, so I exported the library and have nearly finished converting it all, but I have just noticed that the original library now appears to be populated with playlists I was using 6 months ago and my recent ones have gone.
My 1st thought was that I had copied an earlier library across but even the copy contains songs purchased last week and why would exporting it change the playlist content anyway??