Apple has now made it possible to upgrade songs à la carte to iTunes Plus. You pay the same fees (30¢ a song, 20% of an album's price etc) and simply pick and choose which iTunes purchases you want to upgrade. You'll need (at least for now) to be using the 1-Click iTunes purchasing as the Shopping Cart isn't working for upgrades, however if you've been yearning for DRM-free upgrades but can't stomach the large library-wide upgrade fee your prayers have been answered.

Source:→ TUAW