UIRoot, when "automatic" is turned off, will scale the UI making it look exactly the same size regardless of the resolution.
UIAnchor lets you attach things to sides and corners of the screen.
UIStretch lets you stretch the UI or just UI widgets, should you have need, based on the size of the screen. It can do that based on width + height, or just height alone.
UIAtlas has an option for pixel size and reference atlases. Reference atlases are dummy atlases that point to another atlas. You can switch the pointer at run-time, swapping SD for HD easily. As you do that, make sure to set the pixelSize property -- if it's '1' for your SD atlas, set it to '0.5' for your HD atlas (assuming the sprites are double in width/height), and turn off UIRoot's 'automatic' option.