A) Depends on how your chain was size in the first place. If it was sized via small-small (chain as long as it can be) you have nothing to worry about. If it was sized big-big (as short as it can be), you may have issues running cross-chained gears depending on how close they sized it.

B) Wear. How many miles are on the old chain and cassette? You may need or want a new chain due to wear anyway.