Do these manuscripts change Buddhist history?  Well to most of us in "the West," NO, because we just don't know very much about Buddhist history.  But maybe we should.  (Here's a brief piece from the British Library on the Ghandaran mss. in their collection.)  And maybe to those who are scholars of the tradition, or devotees, or even just a little bit informed, maybe they do.  Interesting to read this, anyway:

The discovery of previously unknown texts also offers a hint of how much of the Buddhist literature that once existed has not come down to us. The fact that extensive remnants have come to light in Gandhara is no coincidence but rather a result of particular climatic and cultural factors. Gandhara lies beyond the central monsoon zone, whose extremes of heat and humidity prevent the longterm survival of organic materials such as birch bark or palm leaf. Additionally, the Buddhists of ancient Gandhara had a practice of ritually interring their manuscripts in clay pots or other containers in the precincts of their monasteries, further promoting their preservation. It was likely due to these incidental factors that the oldest known Buddhist manuscripts were found in Gandhara, and not because such manuscripts were unique to the region. Similar texts must have existed elsewhere—perhaps everywhere—in the Buddhist cultures of the Indian heartland, but there is virtually no chance such manuscripts would have survived the deleterious effects of the monsoon climate.

    The discovery of some random fragments of the literature of Gandharan Buddhism from the beginning of the Common Era is significant in part because it enables us to triangulate with the Pali and (partial) Sanskrit canons and begin to see all three as merely the surviving fragments of a vast tapestry of local Buddhisms and Buddhist literatures. Even from the tattered remnants of this grand tapestry, we can discern common threads in the form of shared basic texts, particularly among the sutras recognized, at least in theory, as authoritative by all schools, which still form a common core of beliefs and principles.

    But we also find differences—sometimes minor and technical, sometimes significant and surprising—among the texts of other genres, many of which seem to be locally composed materials: commentaries, scholastic treatises and debates, local stories, hymns of praise to the Buddha, and more, which together comprise as much as half of the Gandharan manuscript material. In short, we find a shared conceptual foundation on which the various regional and sectarian traditions have built their own superstructures. Some of the differences are merely formal, for example in their differing formulation and arrangement of the materials, while others are more substantial, as in the Gandharan reconception of the Vessantara story.…

    …it’s not helpful to think of Buddhism in terms of a contrast between a single original source and the implicitly inferior derivatives of that primal source. Rather, the complexity and variability of Buddhist teachings appear to have been built in from the very beginning; after all, one of the Buddha’s special qualities was said to be his intuitive ability to adapt his teachings to the capabilities and needs of the person or persons to whom he was speaking.

For people like me, obsessed with "origins," stuff like this is fascinating, in no small part because it vexes our reflexive obsessions, and makes us think in more complicated ways about "beginnings".  As we should.