I've also downgraded when travelling with friends/family in a different class of service. The gate agents will do it if the flight is not full. The purser on mainline flights has the discretion to move you around the aircraft if requested. I often travel in eco on the feeder and my family in C on awards as I only upgrade the longhaul with vouchers. Reuniting the family in C or Y has never been a problem for the purser.