Converting purchased ACC files in iTunes

I have iTunes on a Windows 10 PC. In the past I have always converted/duplicated purchased songs to AAC as a back-up I store out of iTune on a HD. In the past when I choose a purchased song to convert the only choice allowed was AAC. Always worked. A few days ago I tried to do a conversion and now the only choice I'm allowed is WAVE. Don't want that. How do I change my choice back to AAC?