MMA legend and old school BJJ black belt Enson Inoue has been retired from MMA since 2004 (he had a short comeback fight in 2010), but he has started training Jiu-Jitsu again in 2016 after taking a 10 year break from the sport.
He actually went through a weird period when coming back to Jiu-Jitsu as he found himself lost with all the new techniques. After deciding to demote himself to purple belt, and creating much buzz in the BJJ community, he then decided to go back to black belt rank. Inoue was a BJJ black belt since 1999 and had started training in the mid 1990’s under Relson Gracie and then in Japan:
In this recent video, Enson tells the story himself and what pushed to take that decision: