How do I convert previously downloaded music to itunes plus (non-DRM music)?

iTunes Store: iTunes Plus Frequently Asked Questions (FAQ) - http://support.apple.com/kb/ht1711 - extra tips at: https://discussions.apple.com/message/20001723

10/2012 post saying iTunes Plus option is possibly no longer working or at best erratic - https://discussions.apple.com/message/20050696

18/11/2012 post saying the iTunes Plus program ended and suggests one option using iTunes Match to upgrade.

Don't forget, you get a limited number of burns of DRM music to CDs which then behave as usual.