The challenge can be revisited by selecting the ilvl option that syncs the party to the minimum ilvl requirement of the dungeon.

That said, keep in mind that this is a vertical progression MMO - they want players to get and keep current on the content, not linger behind at old bottle-necks beating their heads against the wall. Yes, that means losing some of the past challenges but they seem to be taking steps to allow players to revisit those challenges, perhaps in ways they never really cleared it before.

So yes, there needs to be a catch-up lane for those new to the game trying to get to the point where they can play alongside the friends that invited them. There's no point in making them suffer for year-old content when they're trying to get current.