The forever stamps will be good "forever"....so no matter what price you bought them at, they will be good. Stamps could go up to a $1 tomorrow, but your original forever stamps will be OK to use without any extra postage.
From usps.com
On May 12, 2008, in Washington, DC, the Postal Service™ re–issued the non–denominated First–Class Mail definitive Forever Stamp™ (Liberty Bell). The stamp was designed by Carl T. Herrman of Carlsbad, California. The stamp was first issued in 2007.
The Forever stamp will always be valid as First–Class postage on standard envelopes weighing one ounce or less, regardless of any subsequent increases in the First–Class rate. The stamp art depicts the Liberty Bell, which is perhaps the most prominent and recognizable symbol associated with American independence. Over the years, the historic significance of the bell’s message has transcended our national borders, and today the Liberty Bell is an international icon of freedom. Replicas of the Liberty Bell exist in all fifty states and the District of Columbia.
The stamp art features a computer–generated image of the Liberty Bell by nationally acclaimed artist Tom Engeman of Brunswick, Maryland. His previous projects for the Postal Service include stamped cards for Carlsbad Caverns National Park (2002), Ohio University (2003), and Columbia University (2003), as well as the 2003 American Eagle definitive stamp, and the National World War II Memorial stamp issued in 2004.