Hey Robotech_Master,
Thanks for the question. At this time, the most applicable way to upgrade your previously purchased music is to subscribe to iTunes Match:
iTunes Store: iTunes Plus Frequently Asked Questions (FAQ)
http://support.apple.com/kb/HT1711
Can I upgrade my previously purchased music to iTunes Plus?
Yes. If you subscribe to iTunes Match, you can download your previously purchased music again in the iTunes Plus format. To do so:
1. You need to delete the original DRM song from your library first. Doing this enables you to download the song again in the iTunes Plus format from iTunes in the Cloud.
2. Click the Purchased tab in the iTunes Store.
3. Click Music.
4. Locate the artist of the song you want to upgrade.
5. Click the iCloud Download icon next to the song you want to upgrade.
Thanks,
Matt M.