If you i.e have a rebar set consisting of 10 bars with a length of 10 meters and you want to change the number of bars to 12, and change the length to 12 meters. You can't control what end of the bar will be expanded by 2 meters, and you can't control at what side the rebar set will expand by 2 bars.
You should be able to control at what direction the rebar set expands without having to mirror the whole set. For example like "flip rebar set". Same should be applied to the bar length, so that you can control at what end the bar will expand if you change the length in the properties window.