For starters the "idol" you see is a character. Stage names, falsified ages and wiki pages, plastic surgery and etcetera. The average human being couldn't handle hours of practice daily, photoshoots, performances, promo, recording and cross promotions all while there is little to no time for food or sleep. They would fall apart.
So there very well could be 2 or more people playing one character. They get different beauty procedures often so that's the excuse as to why they look different. I believe when Michael Jackson had the dramatic change they swapped the original out for a new one. The world is a stage.
