You can convert a song to a different file format while keeping a copy of the original. For example, you can save a copy of an uncompressed song file such as AIFF or WAV to a compressed format like MP3, AAC, or Apple Lossless Encoder.
Saving a copy of a song in a new file format

When converting from a compressed to uncompressed file format (for example, from MP3 to AIFF) you shouldn’t notice any reduction in sound quality. However, when converting between compressed formats (for example MP3 and AAC), you may notice a reduction in the sound quality. For the best results, if you want your music encoded in a different file format, you should import the music again from the original source using the new encoding format.  Read more... (548 words, estimated 2:12 mins reading time)