I am not sure how to do what you want, but what I have read. Changing from mp3 to AAC wouldn't buy you any thing. The mp3 format is more conpressed than AAC. So moving from AAC to mp3, there is a loss of sound quality. Moving from mp3 to AAC, will not get that sound quality back since it is lost already. You will only hear this if you have a very good ear. Most of us wouldn't know the difference in sound quality.
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