I ran a game in 3rd ed which had the real world time limit of "This game ends when 4e comes out". The PCs were working to fight off an interplanar invasion/giant planar conjunction by gathering the Rod of Seven Parts. When the main PC gathered the last bit, she ascended into godhood, fought the Queen of Chaos into the ground and kept the universe from exploding.
The next campaign was in the same setting, but twenty years later. As a result of the epic planar realignment that ended the 3rd ed game, the 4e rules were in effect. Some characters were majorly effected, some were not at all. Elven society was in chaos for a while, because all of a sudden everyone had to start sleeping every night. Most elven houses didn't even have beds.
(Several other games I played in transitioned without it being addressed fictionally at all. One session we had certain abilities, and in the next we had different ones, but we assumed that they were roughly the same abilities but just represented differently mechanically.)