Original recordings are made with many more "tracks" (channels) than the 2 (or 5 or so) that exist in media that is sold to the public. The multitrack masters are mixed down to 2 or more channels. If the original recordings still exist, they can be remixed in different ways to yield a different final result starting with the same old material.
Two interesting remixes that I know about are the sound track for the movie "Sweet Dreams" (life of Patsy Cline) and the DVDA "Emmylou Harris, Producers Cut".
When Patsy made her recordings, she was in an isolation booth, wearing headphones so she could hear the musicians, and the track of her voice, although old, is of excellent quality. This track was remixed with some of the old accompanyment tracks and some newly recorded tracks to yield a final result that is better than the original releases.
Most of the Emmylou Harris recordings are also very old and were never intended to be more than stereo, but with many tracks available, a multichannel remix could be done. By the way, this DVDA includes a video interview with producer Brian Ahern which discusses in fair technical detail how and why the remix was done the way it was ... very interesting