I was hoping they'd take the FFXI route with this and make the old event gear re-obtainable during the event by quest or buying it from an NPC during the related event. As far as achievements go, they could easily have it where you only obtain the achievement if you acquire the gear during the original event it was released in, an example is how if you buy the Easter egg hats from the market ward, you do not get the related history tab achievement from the event.