In a recent and exclusive interview with MTV, Potter producer David Heyman confirmed that Dan, Rupert and Emma are slated to appear in the Harry Potter and the Deathly Hallows epilogue and were not going to be replaced by older actors:
"We will shoot it,” (the epilogue) producer David Heyman confirmed for MTV News. Rowling wrote a flash-forward ending to “The Deathly Hallows” that shows where the characters end up as they grow older. Heyman also indicated that the original Hogwarts actors would be filling those roles for the sequence, rather than being replaced by older lookalikes.source: mtv.com
“Yes,” he acknowledged, nodding that aging done to the characters would use the existing cast."