You should keep the old one in a safe location as it is Disc one of the backup set. Each additional disc is added to the set and would possibly be used to perform a full restore.
The only time you can get rid of the old discs is when you have performed a new, full backup starting with a new disc one and you are sure that you no longer need the older versions of data on the other discs.
-steveMicrosoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator