http://www.moneyfactory.com/newmoney/main.cfm/media/features50
Features of the new $50. “Your old money will always be good. Every U.S. currency note issued since 1861 is still redeemable today at full face value.” It’s rather brave to guarantee that all notes will be valid for all time, I think. Also odd that this—the official site of the U.S. Bureau of Engraving and Printing—is a .com.