Apple has apparently bowed to pressure because the iTunes Store is now allowing upgrades to iTunes Plus on a per track basis. Back when Apple first dropped DRM on some of their music they started allowing customers to upgrade their previously purchased tracks to the higher bit-rate and DRM-free iTunes Plus versions. Unfortunately, until recently Apple required you to purchase the upgrades for all of your qualifying music all at once, whether your wanted them all or not.
Now it seems you can upgrade your previously purchased music to the iTunes Plus versions for 30 cents per track. The albums in my account seem to cost between $3 and $4 to upgrade. You can find all your qualifying music on the iTunes Store by following this link (iTunes link) or from the iTunes Store home page.
It's nice to see that Apple has seen the light on this. Note that Macworld is reporting some kind of glitch with using the shopping cart and are saying you need to turn on the buy with 1-click option in the iTunes Store preferences to get the correct upgrade price (and not be charged the full album price again).