This is my understanding of how the shift occurred.
The original X (chi) was where all the events leading up to the Keyblade War occurred. At the end of that game was the Keyblade war in which the five unions fought and the dandelions fled to "the outside world". All the wielders who fought perished in the battle (except for player character who is rescued by Skuld and Ephemer). And the Dandelions end up trapped in the data version of Daybreak Town without realizing it. At this point, the Dandelions, except for the new five union leaders, are put into a sort of sleep or dream state where they relived the events of chi. This is where Unchained begins. So essentially everything experienced in that game is actually happening in the realm of sleep as a way of overwriting the events of the past. That is when the memory wipe occurs. This continues up until the point when the war should have happened, but of course the memories of the war actually happening are removed from the rewrite. Once this point has been reached, Union cross begins, which can be considered the era "post war". And that is where the story is now.
TL;DR
Chi was that actual war, Unchained was the memory erasure and replacement, and Union Cross is the events after the war.
Hope this helps