Is the spline shape created with a spline sketch? If so, you can set the spline to Proportional (in the spline propertymgr toward the bottom), and just make sure the endpoints of the spline are driven by the same amount in the same direction. Then you don't have to worry about the shape changing, or the internal control points.
Edvard, not sure what you're trying to doing other than make different sizes. You can make different configurations where the length, width and height changes. What Spline are you referring to?
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