Is there a way to change the format of files downloaded from iTunes Store?
You have 2 big problems with files downloaded from itunes store. First, i am pretty sure you cannot convert these to a non-DRM format (e.g., mp3, wma) b/c itunes wants you to use the files ONLY in itunes or on an ipod. If someone has figured out how to break these restrictions, i haven't heard about it. Second, the file you downloaded from itunes store is almost certainly compressed to 128 kbps. That level of compression, by definition, DESTROYS any hope of a decent sounding file.
Bottom line is that itunes store is almost worthless if you care how your music sounds.