The Swormage has the 7th level Encounter Power Spikes of Agony. This Encounter has an effect that includes the At-Will Aegis of Assault. When attacking with Aegis of Assault you normally make a melee basic attack, however Spikes of Agony says that you can use it in place of Aegis of Assault's melee basic attack.
Can you replace that melee attack every time you use Aegis of Assault, or, because it is an Encounter Power, can you do it only once per encounter?