Greyhawk will most likely get updated a few years from now, like the other setting books when they get around to it for
4e. But like Siani said, the CRT won't change due to 3rd edition, 3.5 edition, Ars magica, 4th edition, or World of Darkness edition or any other type of edition until she feels like she wants to update the setting.
To introduce tieflings is easy enough to a xenophobic setting. They're going to be pariah and freaks. Shunned, insulted, tossed rotten veggies at, and probably live down the river in a small hut and raised by parents who either don't want them, are mad, or generally going to have a hard life. They're child hoods are going to be hellish (no pun intended). If your going to go with the
4e idea on their other worldly origins, it is easy enough to do some research on the Greyhawk setting to note that there are a few fallen evil kingdoms in the past that probably dealt with devils (Vecna, Invid, etc).
As for Dragonborn, the people of the region are going to have problems due to mistaking these people for lizard folk. Which are abundant in the swamps surrounding the area. But if you want to play
4e Dragonborn, it could be explained to a massive migration from the West, emissaries from the Blood Isles of Io, or etc.
But if you plan to play any of these races, the setting of CRT states you need a disguise. So invest in those hats of disguise, spells of disguise, or the engimatic black knight that never takes off their helmet of King Arthur Lore.
The
FR rooms don't need to change, unless Nashira feels like she wants to update the room. But with all the planar activity, its easy enough to play planar travelers from an alternative
FR (cue 50s creepy music and the quote "I come from the Future!") and is now trapped in the past. And Waterdeep is a major trade city that, depending on what Ward your in, a Dragonborn and a Tiefling will probably just earn you a odd look.
As for Sigil, anything is open game.
And on the side, I like
4e. It feels like
3e when it first came out. And like
3e when it first came out, requires alot of House Rules and things you cannibalize from the previous edition. Until the splat books come out and spoil you with more options. (2e - race and kit books.
3e - races and class books. And the trend continues further in
4e).