Kudos to Apple for finally doing this. There is no reason to have to pay for a song twice if you tried a track and then decide to buy the whole album. I had about 40 albums from which I bought songs but not the whole thing. I only “completed” one album, the other I wanted said it was being modified and to comeback later, but I also bought several songs from albums I’d previously purchased a song from. The reminder of the songs I’d bought was a nice tickler to look over the bands again and listen to the snippets to see if any other songs struck my fancy.
To find this new feature just go to the itunes home page at the iTunes Music Store and look on the right side in the “Quick Links” section and go to “Complete My Album”