If you offset a polyline, then all parts of the polyline will offset. In order to offset only one element, you would need to explode the polyline first. But rather than doing any of that, use the stretch command instead. This will move the segment in question, while also stretching adjoining segments to match. Do a crossing window that includes the endpoints of the segment you want to move, then move it. All other defining points for any other segments of the polyline not included in the crossing window will remain unchanged. You must pick the end points, or nothing will happen. Think of it in terms of how a line is mathematically defined. Stretch is a very useful and powerful command. Come to think of it, you can also use the grips, which are the blue squares you see in your 1 x 1 square example above. If you click on a blue grip, you can then move it, which in turn stretches the entities that are associated with the grip. If you select the midpoint grip, it will stretch both end points of that segment.