The big problem in FR wasn't the advance vs. reimagining. It was the catastrophic
change. You can't stop that when the changes are as big as the 3e -> 4e changes.
So say, instead of creating a world shaking event to include dragonborn, just treat it as a fresh start and write them in as through it were a new campaign setting without it having to go through some traumatic happening.
Obviously, the problem there is that these foreign elements obviously don't belong, and haven't been important. Instead of making something new, you're trashing all the stuff that came before, which is also a problem.
Really, if you want a great diversity of interesting settings that aren't shoehorned into changes, you have to do something that WotC won't do: let the setting trump the edition.
It doesn't have to be in a huge way, but when the two conflict, the setting should take precedence. No dragonborn in FR by default? That should be
totally fine. Give the DM's some interesting ideas on how to integrate them if they choose to (including "they come suddenly from offscreen" and "they were there all along" options) and leave it the heck alone. It's not like people are hurting for racial choices, especially given FR's insane diversity of demihuman sub-races.
4e has fetishized the cross-compatability of its rules elements to an absurd degree, and rendered them sacrosanct by the "everything is core" dogma. Which is also weird for an edition that wants to publish a lot of settings. Settings SHOULD be different. My New Crobuzon game should certainly not have the same races, classes, and archetypes as my Middle Earth game. Dark Sun and Forgotten Realms are
different and this shouldn't be a problem.
Fortunately, I think WotC has learned a bit of a lesson after FR. Eberron wasn't blown up, which was
great, and other bog-standard fantasy settings probably don't need to be blown up, either (Dragonlance and Greyhawk are probably flexible enough to accomodate tieflings and dragonborn and assorted weirdness).
They're not abandoning their ethos, but as long as they stick to Generic Fantasy Land #324, or create new settings, there might be relatively few conflicts on the horizon. Heck, they've got two probably-dynamite settings they're just sitting on the IP for (the runners up in the setting search), both of which probably share some Eberronian elements (such as easy expandability). That's at least four more years of settings that won't cause as much wailing and gnashing of teeth as FR did, not to mention new or re-imagined cultural settings. "Oriental Advenutres" and the like, where you can include everything by default and dress it up in funny clothes and everyone's happy.
4e doesn't need to do settings that are "limited."
What kind of concerns me is if they want to -- the Dark Sun buzz, for instance, might tempt them to do a setting that doesn't have a whole lot of business having certain default D&D elements thrust into it.
Advance it? Re-imagine it? It doesn't matter. If we've got gnomes and woads and tielfings scampering around Dark Sun, it's going to be dissonant, and you're not going to get the effect you really want out of playing that setting, no matter the justification you use for putting it in there. It doesn't belong. That's only a problem if you're sycophantic about turning everything into a kitchen sink setting.