The fascinating thing about the Poland example is that the Germans living in modern fay west-Poland were moved out and into what's now Germany. So the culture and people are gone. What you see are the political effects of better infrastructure.

Sort of, but it is more like echoes. In the first and possibly most well known case the previously Prussian areas of Poland had all of the ethnic Germans forcibly removed. Because of this abrupt and extreme change there must be something else going on since the culture and values in fact changed quite suddenly and yet differentiation endures.

True, but they do change; if e.g. an empire hangs around for long enough, the original culture will be lost or assimilated. See for example Christianity throughout Europe, plenty of examples of erased or merged cultures to the point where nobody knows what happened before. Or the lasting influence of colonization throughout the world. Or the change of culture and language that the US has throughout the Hollywood media-consuming world; listen to random people in western Europe and count how many Anglicisms they use in their native language.