The answer to this is somewhat complex, as it does depend on so many variables. 2 pax changing (pre take-off) from row 25 to 28 on a B747 will have no significant impact. Two pax changing from row 16 to row 2 on an EMB-145 (or similar commuter a/c) will have a significant and potentially serious impact. I am troubled (i.e. worried) when I see cc disregard seat changes on smaller a/c.