You can convert a song to a different file format (and keep a copy of the original). For example, you can save a copy of a compressed song file such as MP3 or AAC in an uncompressed song format (AIFF or WAV).
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 might notice a reduction in the sound quality. For the best results, if you want your music encoded in a different file format, import the music again from the original source using the new encoding format.
Convert a song’s file format
Choose iTunes > Preferences, click General, and click Import Settings.
In the Import Using pop-up menu, choose the format you want to convert songs to, and click OK to save the settings.
Select one or more songs in your library and choose File > Create New Version > Create Format Version.
The song in its original format and the newly converted song appear in your library.