You can also burn the tracks to an Audio CD then reimport them.
First you need to enable the iCloud Status, Bit Rate and Kind columns so you can see which tracks are "matched" and which are "uploaded." Any tracks that are "uploaded" will not be upgraded. To enable these columns switch to Song view then pull down View > View Options and select the appropriate check boxes then close the small window. Once this is done you can see the tracks that are protected and matched. These tracks can then be deleted from the iTunes library and the HDD (but make sure you have them backed up first, just in case) then downloaded from the cloud. They will then be DRM-free 256 Kbps AAC files.
As Jim points out you will not get a 100% match rate. The process simply isn't that good.