I've been offered Y-J upgrades on EK from BHX/LHR - DXB for £250 in the past.
I've also seen EY's Plusgrade system for myself. On a recent flight from LHR-AUH I was on a paid J ticket, and was offered the chance to bid for an upgrade to F. I declined, but once I was on the plane both premium cabins were full. I can only assume that most of the F seats were taken by people bidding for upgrades from J, and the J cabin was full of Y pax upgrading. This was further compounded when I noticed that the first section of the Y cabin (well over 100 seats?) was totally, totally empty.