Two phrases come to mind, having read every post thus far:
"The Realms are Dead. Long Live the Realms."
and
"To save the [Realms] it became necessary to destroy it."
(Can't recall who made the latter quote (about a village in korea or veitnam, as I recall), but it seems to really come close to the meat of what they are doing to the realms.
In the end, they are removing most of the NPCs (and deities, although technically they are also NPCs, just at the other end of the scale), utterly redefining how magic works in a setting where magic is everywhere and of high level, re-defining the cosmology (again), and moving the setting far enough into the future that few of the actual kingdoms (and even longer lived NPCs) will still be around.
So, in basic, the (physical) map is the same, but the cosmology, deities, magic system, (recent) history, organizations, NPCs, and so forth are all different.
...
I agree with the quoted poster. What remains may superficially *look* like the realms (at least in terms of the map and perhaps a few NPCs and deities), but in every other respect it is a completely new setting.
Perhaps instead of "Forgotten Realms" they should refer to it as "Re-Discovered Realms" or even "Reincarnated Realms." The latter is especially good, as you can come back as a different animal entirely from what you were the first time around, yet still in some respects remain you.