the video only confirms the first part of your post (the border part)
but compleatly disproves the second part (that national identities can change easy)
a national identity is a bitch to get rid off, look for how long Greece, Bulgaria, Romania and Serbia were under Ottoman control, if changing national identities were so easy they still would be, but they aren't, why? Beacose of nationalism, even through all that long occupation they still saw themself as greek, or serb etc
look at the spring of nations, rise of nationalism in teritories that have for hundreds of years been under a foreign rule
a national identity is probably the hardest thing to get rid off for an occupying power