FYI: I just got this expansion and the cards backs have a much brighter green than the original. It is unfortunate, but they are easily distinguishable from the backs on the I.V. edition. I don't sleeve any of my games, but I think it is the only way I can think of for these to work (other than dealing off the bottom of the deck).
I would think CGF and BGG should make this obvious for those who were intending to add these to their existing decks.
Edit: I have the older I.V. printing that came with poker chips.
Not only that but the cards are slightly smaller. I had to buy sleeves with colored backs.
NOTE: You will be in trouble fitting the cards from IV to the new MtG sleeves that are slightly smaller. They barely fit and i slightly bent two cards trying to put them in these sleeves.