True but still the same world. Your looking at 19 years silvermoon was never a frontier city, but a city state in the wilds. It was always a major city, just without a kingdom. but in 19 years it went from a city all alone to the capital of a fledgling nation:that does not make it a new world. The world is the same place it was in 1355 but by 1374 things had changed
Look at it like this in 1760 America was colony's , points of light in the wilderness, by 1780 it was a young country. Still the same world as it was in 1760 but now the area had advanced. The in 1792 the us had 15 states, yet by 1821 a span of 29 years there were 24 states. Even if we keep it within 19 years your looking at 2 more states a large incress in size.
If ya look at the realms history ya see nations rise and fall in 20 years, none of that makes it a new world but does make it more "real"
Okay, so you are thinking about growth and change? I guess it's still a fine line we will have to agree to disagree!
In 1355, going to Silverymoon would be an adventure unto itself based on how FR5 portrayed the world. Orc tribes, barbarian tribes, goblins, trolls, and other monsters were out there, in the open, ready to attack.
By 1374, in 3.X FRCS, where are the barbarian tribes? I think they have been pushed further north of Silverymoon. The orcs have a nation. The trollmoors are still dangerous but only if entered, unless they are out attacking. In this year, traveling from Waterdeep to Silverymoon would be relatively fast and safe compared to 1355. To me, that's a different world.
And it's to that point that I don't see the new FR of 4E much different. Change happened. The cities are all still there. They just let the orc tribes and barbarian tribes back in to play!

Think of how much change we are arguing in twenty years game time and now multiply that by five!
(I don't have my FRCS 3.0 to check what it says about the barbarian tribes.)
I know you didn't say this. I am. But, it seems to me that the Americas in 1500 and the Americas in 1600 are VERY different! That's how I see the changes in FR.
Don't get me wrong. I don't prefer it as a way to explain a new edition. I do find it too bad that the rules are forcing the changes. I can life with it, though, because I do like 4E.
edg