I imported a bunch of CD's using the wrong import setting. What now?

You can adjust your imports settings to AAC (iTunes Plus) and convert the tracks you have imported. You can use iTunes to convert any copy protection free songs in your library another format. When you convert a song in iTunes it will be converted to the format that you have set to import your CDs in. Converting creates a second copy of the song in the format you have chosen which gives you the opportunity to either keep or delete the original. In iTunes go to Edit>Preferences>General and click on Import Settings,, Change your Import format to AAC Encoder for example and choose the Setting or "bit rate" you want (the default setting iTunes Plus will give you a bit rate of 256kbps). Go back to your library and highlight the songs you want to convert and go to the Advanced menu at the top of your iTunes window and choose "Create AAC version": iTunes: How to convert a song to a different file format

If the tunes are from the iTS and Fair Play DRM protected then you can only convert them to an AIFF file. What you posted was that you ripped from CD's to AIFF - those files shouldn't be DRM protected.

MJ

Songs from the iTunes Store are already AAC, so you don't need to convert them.

I suggest you make a smart playlist as follows:

Match the following rule:

Bit rate is greater than 321 kbps

That playlist should contain nothing but uncompressed files. From within that list you should be able to convert songs to AAC without encountering any iTunes purchases (or any songs you've already imported with compression.