I updated some of my old homebrew (used extensively in 2nd, off-and-on in 3rd) to conform to the default assumptions. For example.
I shook up the default pantheon (something I tend to do often, often ret-con style) so conform with the portfolios (but not the names nor necessarily the alignments) of the core deities.
The feywild mysteriously became coterminous with the material plane again after centuries, and eladrin appeared. They offered succor to the elves of the land, and many (mostly old gray elves) went with them. In a 100 years, they formed the current eladrin race (and those elves who stayed behind became the 4e elf race, but are closer to a high/sylvan mix).
My current cosmology was superficially similar to the default D&D one that only a few other nip/tucks (where the Abyss was, elemental limbo) didn't matter. Using the cosmology as-is, with different outer-planes to reflect my deities.
dragonborn came sailing from east, refugees from a fallen empire. Beyond no dragonborn/tiefling wars, they are pretty much as is.
I had a race of undead-human hybrids magically created by an insane necromancer who fancied himself a god. It got little play (one PC, and not for long) and never was resolved. It took nothing to change "undead" to "devil" to account for the new tieflings. Surprisingly, most of the planescape-like tieflings we had fit the general description of the current tieflings (horns, tails) so only a few tweaks were needed to account for them.
The backstories of many monsters didn't matter much; giants never got much play before, so the giant/titan thing wasn't a bother. The angels vs. devas thing was similar.
Lastly, I had always intended to "PoL" my world, so I took a 125 year jump (oddly I created the idea BEFORE I saw the Realms was doing the same) to smooth over the world. I did a "almost cataclysm" which left the world intact but fallen, and allowed my to implement some of the above changes.
Generally, I was happy with the 4e changes to the default and they fit my world well. Too bad my players disproved of 4e's actual play...