iTunes plus is not available for all of my songs

I recently purchased an iTunes plus upgrade for songs purchased before 2009. I converted about 15 songs in total. If I search my library there are still some DRM proteced m4p files lying around which cannot be converted. iTunes store reports that there are no more songs to be converted in my library. If I go to "get info" it says "Kind: Protected AAC audio file". If I go to "Create AAC version" I get "XXX could not be converted because protected files cannot be converted to other formats"

If I can't convert these in iTunes (Windows) is there a reliable way of doing this? It seems very cumbersome to burn the songs to disk and I suspect a lot of metadata will be lost. I wish to use the tracks on a DLNA compatible server (Twonky)