Or leave Normal and Hard as is (after making Shroud Normal a touch easier) and make the Elites across the board harder (even Proof Is In The Poison is simple on Elite at level with a good group - but maybe In The Flesh can be left alone on Elite...), keep some of the Epics as they are so new 20s can begin somewhere, and make a bunch of the Epics harder.
Provide content difficulties which will cater to all players. Make low/mid level Raids possible for people to run on Normal/Hard while levelling but again the Elite versions could be made harder so the higher level guys can have access to more challenging content.
Sure, the buffing of Elites will make the 1st life players who think they can Bravery Bonus their way through the game think twice (or TR groups not ever want to invite 1st life players) but you can still Bravery Bonus your way to 20 easily on a 1st life going for Hard at level and occasionally Elites at 3+ for the 80% boost.
Its not hard, its just a lot of work (requires proper mapping of all the quests for all difficulties and taking in to account reviews by new and old players, well geared and poorly geared toons, etc.).
And maybe alter loot tables across all dungeons for all difficulties too. Though this may encourage people to run content beyond their capabilities. But I guess they will learn quickly enough what they can and cant do.