[I am trying to explain what I think is a conceptual confusion by the "Open Borders" people. Unfortunately, my brain isn't very clear either.]
Land can't move, of course. But borders can. We can't move land across the borders, but we can move borders across the land.
So if half the people in country B want to move to country A, there are three ways we could satisfy their desire:
1. Move the people across the border.
2. Move the land and people across the border, by letting country B annex half of country A.
3. Move the land and people across the border, by letting country A annex half of country B.
Which of those three would be better? Are all three equally feasible? Why would people object more to some than to others? Why would one be more morally acceptable than the others?