This would definitely happen in isolation, not quite as fast as in one generation, but rather over 4-5, if humans are anything to go by. Look at our own society for example. If people had to suddenly start living and working the way their ancestors lived a mere 100 years ago, it would be quite a shock to them. Or when most immigrants no longer know anything about the culture of their ancestors after 2-3 generations. That said, as fast as people forget, they learn even faster. Already the first generation born after the transition (descendants of those who survived) will be completely adapted to the new situation. For a historical example, consider how the English language came to dominate the area of England. When Roman Britain, after two centuries of turmoil, finally collapsed in the wake of the Justinianian plague in the mid-500s, the area where the Germanic dialects that would later turn into the English language were spoken suddenly expanded from a narrow area hugging the eastern British coast to almost touching the border of present-day Wales and Cornwall. Within two generations this shift was complete, with barely a trace of the Celtic languages left in the area now occupied by the emerging Old English language. The reason for this rapid shift is not because the Germanic settlers (the Angles, Saxons, Jutes and Frisians) had the numbers to flood the British population (in fact, the population of Britain, the Romanized Celts, outnumbered the Germanic invaders more than 10 to 1), but because they had a simple, autarkic, yet militarily competent society that could function on very meager resources, which the sophisticated society of Roman Britain, dependant on a surplus of food and labour, Roman know-how and trade with the rest of the Roman Empire, no longer had at its disposal. The Romanized Celts found themselves under Anglo-Saxon lordship, and eagerly discarded their collapsing social model for the social model of these new elites. This went so far that not only they forgot their Celtic tongue, they also discarded their religion, Christianity, in favor of Germanic paganism (later on the English were baptized by Irish and Welsh missionaries).
So, the grandchildren of the Gang of Five would have a hard time without the Great Valley, but after a very nasty beginning they'd be living like the Gang of Five's parents did in the Mysterious Beyond rather quickly.
