Filled Region must have Line Style <by Type> (or default) when sketching, and have the type property option must let you define a Boundary line style to apply by default on that type of Filled Region.
Most of the time you edit boundary lines on filled region you wanna do it for all the lines. Enter to Edit Mode and select the lines way to lose useful time. Specially if you hace dozens of Filled Regions of one type that you want to change theirs boundaries at one, but have to do it one by one or use by override.


Also AutoCAD import hatch must not overdraw ther borders with line of same color (use <invisible lines> instead), there is not necessity of duplicate objects, modifying the borders of these hatch make you lost time. Because you can't Select All Similars on detail lines, you have select all, manually 