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Converting from mp3 to aac will result in even greater loss of audio quality so in my opinion no it is not worth converting them.

The best thing you can do is just re-rip them from CD in aac format.
 
do it if you want. but as Cromulent said it will result in lower quality audio as it is re-encoding an already lossy format (original CD audio was lossless).

i did it ages ago when i was less knowledable on this matter. sure my library went down in size but i then realised it sounds shiat compared to my newer >192kbps mp3s.
 
If you're running out of room you could think about removing the songs/video that aren't used very often. Most people don't listen to every song they own so this might be a good choice for you.
 
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