It depends on which media you are referring and where your media is stored.
Mythtv recordings can be preserved, but it is a huge pain, and I typically do not bother.
Video media depends on where you have put it. If it is on the same drive as your core install... then you will need to move it off, and move it back. If your media is on a network drive, you won't need to do anything. The new install will pick up and go.
You might try the new dvd version... but it is experimental. The newest version is untested, but should it work, should be better than the earlier versions. People have had good success with the version tested in the wiki.http://wiki.linuxmce.org/index.php/Snapshot_overview