When you right-click a song in your library you get a list of actions, one of which reads "Create AAC version" or MP3, Lossless, etc. I believe you can change this in import settings. Go into Preferences then on the General tab there is a bubble that says Import Settings... There you can choose what encoder you want for songs that you import from CDs and, correct me if I'm wrong, it also applies to that "Create AAC Version" option when you right-click a song. This way you can convert your songs to any bit rate and encoder you want