Given that the question is tying game mechanics with in-game events, I would like to offer an answer that follows the in-game history of the setting, and not the real-world history. The in-game history was gradually reconstructed with the past (such as the Arcane Age) written after the "present", which leads to interpretations as in the answer by SevenSidedDie, some points of whom I disagree with. For example, there are data supporting that Karsus's Folly was indeed FR canon during the 2e.
The Arcane Age line of products are something like a mini-edition, it is AD&D 2nd edition with various modifications (like the absence of Vancian spellcasting, which had been a staple of D&D from the first published game). After this event, the game mechanics change to those of AD&D 1st edition. Quoting from the Netheril: Empire of Magic boxed set (page 32-33, emphasis mine):
After the death of Mystryl, ... specialty priests ceased to exist; priests became clerics, ... Paladins received spells from the cleric spell list; rangers received spells from the druid spell list (from the original AD&D game, or simply use the spheres available for druids in the Player’s Handbook).
Same thing is made even more explicit in the campaign expansion adventure, How the Mighty are Fallen (HtMaF). Quoting (pages 53-54, again emphasis mine):
For DMs who desire to be historically accurate, game play reverts to strict original AD&D rules (not 2nd Edition). This means that clerics and druids are two separate classes, as are mages and illusionists. Specialty priests and specialist wizards no longer exist.
... assassins were hired from the ranks of the thieves guilds .... Soon, the assassins had a guild of their own. ... The reformation of the bards ... nearly destroyed the profession. Bards left their dependence on wizard spells behind, instead opting to learn a bit about everything. They became fighters (until 5th level or so) and then became thieves, remaining so until they had achieved at least 5th level in their new class. Finally, they left thievery behind to study with the druids, gaining druid spells as they advanced.
Also known as the Avatar Crisis, or Godswar, this event was used to explain the transition from AD&D 1e to 2e. For example, the complete removal of the 1e assassin class (whose introduction is alluded to in the above quote from HtMaF) from the game was tied to the death of deity Bhaal, whose portfolio was murder. Much like how the How the Mighty are Fallen is the adventure allowing players to experience the transition, a trilogy of modules, called the Avatar Trilogy (Shadowdale, Tantras, Waterdeep), allows players to do the same for this transition.
Circa 1371 DR - Transition between AD&D 2e and D&D 3e
The third edition Player's Handbook was published in 2000. There were still 2e materials being published in that year, and amongst them is the FR campaign expansion Cloak and Dagger, which covers the timeline between 1368 to 1370 DR in its chapter 2; and mentions a few things for the "future", 1371 DR onwards. When the 3e FR Campaign Setting was published in 2001, it stated 1372 DR as its current year (page 78).
This transition is smooth in the FR. However, 2e to 3e transition is often associated with the adventure module Die Vecna Die!, which takes place in Greyhawk, Ravenloft and Planescape campaign settings, the last of which is really a metasetting that includes FR. That adventure ends with (page 151):
Some Outer Planes drift off and are forever lost, others collide and merge, while at least one Inner Plane runs "aground" on a distant world of the Prime. Moreover, the very nature of the Prime Material Plane itself is altered. ... New realms, both near and far, are revealed, and realms never previously imagined make themselves known. Entities long thought lost emerge once more, while other creatures, both great and small, are inexplicably eradicated. Some common spells begin to work differently. The changes do not occur immediately, but instead are revealed during the subsequent months. However, one thing remains clear: Nothing will ever be the same again.
With the murder of Mystra, the Weave of magic unravels. Cyric's madness as well as energies from the Far Realm completely poison the magic. Various calamities take place over some years, partly because various geographies on the planets Abeir and Toril swap places. 4e products start about a hundred years of in-game time after.
The planets Abeir and Toril start to separate. Mystra and many other deities return. In 1489 DR, Ao declares the end of the Era of Upheaval. Players can experience parts of this story with the Living Forgotten Realms adventure The End and the Beginning.