I tie a surgeons loop above my dropper fly. Then put the line through the loop. Then put over the bend of the hook. And pull the slack out to tighten down on the Hopper (Top) fly hook. Making sure it cinches down in the center of the bend of the hook. Then you can have several different droppers ready to go. For quick change.

If you tie to the eye of the top hook. You will have some big knots ! Do not ask me how I know !