According to the AU iTunes store, for AU iTunes users, the cost to convert your existing iTunes purchases to “iTunes Plus” DRM-free 256kbps AAC is:

• $0.50 (inc GST) per song
• 30% of the current album price
• $1.00 per music video (same quality)

From the looks of it, not all music is yet available in the iTunes Plus format.

Thanks to Apple's reluctance to specifically show the iTunes software version on the download page, it's not clear to me if everyone needs to upgrade, or if recent versions can skip upgrading. (I'm running 8.0.1.11 — it appears to be sufficient?) It may also mean problems for people with moderately old (pre-OS X 10.4) Macs.