Assuming they're not protected by DRM you can use iTunes to convert them. Select the MP3 settings in the importing section of the preferences and then use the Advanced menu to convert the tracks. This'll make a copy of each selected track at the bitrate you chose.
No matter which option you select, the resulting song file will be of lesser quality than the original. If possible, re-rip a CD as MP3s rather than convert the already compressed AACs.