Here is a summary of the lastest instructions from Apple(1) on how to convert a song’s file format for free using iTunes…
To convert a song’s file format:
If you haven’t imported some songs into iTunes yet, you can import and convert them at the same time. This will create a converted copy of the file in your iTunes Library based on your iTunes preferences. To convert all the songs in a folder or on a disk, hold down the Option key (Mac) or Shift key (Windows) and choose File > Create New Version > Convert [import preference setting]. The Import preference setting will match what you chose in step 3. iTunes will prompt you for the location of the folder or disk you want to import and convert. All the songs in the folder or on the disk will be converted.
Need help? For assistance please call us at 1-877-872-2060 (USA/Canada) or +1-404-869-0701 (international).