From Apple‘s iTunes Plus FAQ
Can I upgrade previously purchased music to iTunes Plus?
Yes. Any available upgrades will be show on the Upgrade My Library page (Music received for free is not eligible for upgrade). You can upgrade all music at once by using the Buy button…You cannot choose which songs, music videos or albums to upgrade individually…
Look, I know it’s only about $4 to upgrade my library, but I want to upgrade one Goldfrapp album, not a David Bowie track I forgot I bought 3 years ago.
Plus, why would I want to buy albums from iTunes right now when I know that they will eventually be available in iTunes Plus? Why should I spend another $3 to upgrade that music when I could wait and pay the regular $9.99 when it comes out?
You might as well buy the CD at that point.