Now when you visit the Upgrade My Library page (iTunes link) you'll see the option to upgrade individual tracks (see Thriller in the bottom of the screen shot above) in addition to the much larger "Buy All" button in the upper left of the page.
It should be noted that the "Buy All" process replaces all music you've purchased on iTunes with iTunes Plus versions of the same music. Macworld also cautions:
If you use iTunes’ Shopping Cart for purchasing media and add an album to that Shopping Cart, it will be listed at full price rather than the upgrade price. Click the Shopping Cart’s Buy Now button and you will pay full price for the album ($9.99 versus $3, for example). For this reason, until Apple irons out this glitch, be sure to set iTunes’ Store preference to Buy and Download Using 1-Click.
More information is in the updated iTunes FAQ.