The replacement always depends on the level of damage and the type of display your device has. Irrespective of damage the device having an LED display need to change the glass only. But, in case of AMOLED to Super AMOLED you are required to change the glass and screen both.
LED screen has a display glass on the top that displays the UI and beneath that screen, there is a tube-like LED light that provides the light to make the UI visible. So, in case of minor damage you are just required to change the upper display screen and not the LED screen beneath it. While on an AMOLED to Super AMOLED there is no backlight thing. They have tiny led bulbs integrated to display the color information as per UI. So if you damage your AMOLED to Super AMOLED, you are required to change the glass and display both as they are combined.