Imagine you're Oracle and you pick early-game. Either the target dies or survives to the lategame where information-finding is less useful. But ok, say the target has died, and Oracle is still stuck, destined to reveal the dead man's role when everyone else knows?
Oracle should be allowed to change his choice, atleast if his target dies before he does. Maybe Oracle should be able to change his decision at any time. But he will still reveal one role and one role only.