My vote would be for an all-new setting, specifically designed for the new system and themes. Anything else is going to be, like the 4E Realms 'update' an uncomfortable fit for many people, and, IMO, still fail to be the ideal setting to highlight what makes 4E new and exciting.
You've made a shiny new dress and you want to show it off. You don't tart up Margarat Thatcher and send her down the runway, hoping that her supporters think she's Kiera Knightley.
If one of the older settings had to be converted, Dark Sun and Scarred Lands both already have a 'point of light' philosophy, but neither needs to be advanced a 100 years. Indeed, as already mentioned, both would be even more exciting if *pushed back* a hundred years... (Eberron set during the Last War, a couple decades back, could also make a neat option, as would a Greyhawk set in the middle of the Wars. Instead of re-arranging the world, work with what is already there.)
After three decades of avid comic-book-fanboy-ism, I've come to think of writers who utterly nuke a character, team or setting as weak in their craft. Because *inevitably* that character wasn't done. That writer was done, tapped out, creatively bored, with that character, but no matter how dramatic and final the death-scene, a few years down the road, someone with fresh ideas and a fresh perspective will come along and say, "I can't believe they thought there weren't any stories left in this character!" and ressurect the character through some dubious contrivance, and then, quite often, proceed to tell the most amazing stories with some character that had been abandoned by writers who thought he was 'written out' or 'finished.'
I feel the same applies to game settings. It's not that the Realms was creatively weak or tapped out or finished off, it was that the current crop of writers were bored with it. To the many fans of the setting who weren't done with it yet, who weren't bored with it yet, who were still writing netbooks full of information that hadn't been detailed anywhere before (proving that even amateurs could find stuff that hadn't already been done!), saying that 'Oh, this setting is finished' isn't proof that the setting was finished, only that the people saying so had run out of ideas or lost interest in the setting.
The *last* people I would want advancing the timeline of (or making other large-scale changes to) a setting would be those who didn't like it the way it was or who was 'bored with it.'
Bored children like to push card-houses over, and tend not to be terribly concerned with how long it took to build, or concerned that they won't be capable of making anything better than what they've casually toppled over.