Maybe this has been answered already, but do these new character decks supersede the original character add-on deck?
No. They are completely different.
The character add on deck(s) is specifically designed to tie to a base set, adding a 4 new characters (with a couple of nice cards designed for them) and a bunch of basic cards to allow the set to cover a fifth & sixth character. There is a Rise of the Runelords one for the RotR base set, a Skull & Shackles one for S&S etc. All the cards are designed to be in from the beginning.
The class decks are primrially for organised play, but can be used for other forms of play. Each deck has 4 characters of the same class, with similar but different abilities. They have cards for all levels of the game (basic & 1-6), allowing you to build a full character from it.