PROJECT POWER CORES AND BLUEBRINTS

Ok im sure many of you got a project blueprint these cost 11 power cores and used to craft a porject skin. And many of you got a blueprint for a skin you already had making the blueprint useless except for essance. SO WHY CANT WE USE ESSANCE TO REROLL THE BLUEPRINT? I mean i have almost every project skin the only ones i dont have is leona and ekko. The blueptint i got was zed now i have 11 power cores and 2 augment chest fragment with nothing to do with it.