Now it’s possible to update your previously purchased iTunes songs to iTunes plus without upgrading them all. In the past, it was all or nothing, take it or leave it, which could prove to be quite costly if you have a large Library of songs purchased from iTunes.
iTunes Plus songs offer a higher bitrate and are DRM free. At $.30 a track, it’s likely worth it for some of your songs. And now, “some of your songs” is an option.