I've had a similar situation. Booked F A380 DXB-BKK-DXB in October, signed into the app a few weeks ago to find the return was now a 2 class 777 in J, no email notification, nothing.
Phoned up and was asked if I was happy with the change......clearly I wasn't so have been re-booked on the only A380 BKK-DXB that day in F unfortunately it's 0300.
Kind of astounds me that having paid for a return F ticket, Emirates thinks it's acceptable to downgrade one leg, not inform customers then assume you are happy to accept it with no compensation. Bizarre.