Why not keep both backups on your USB stick, that way you'll ensure that you have all of the addresses from each time the wallet was backed up, and not risk old addresses not being in the new backup?
I guess my question is I could see myself, quite often, archiving and un-archiving addresses that ALREADY exist in my wallet...so would that backup file from a few days earlier (before I unarchived it) still work? It could be a pain to go back to the safety deposit box every single time.