Well, if your old save was from the same base game version as Expedition uses, you're probably OK. The Expedition content itself is "add-in" to the game, so you don't need to start a new character. I'd probably go ahead, were I you, and make sure your game is using the most up to date version, save under that version, and then download/install Expedition. Just in case, you know? But, short version is, you don't need to start a new game.