Originally Posted by
MSPeconomist
A change in operating carrier (including between different commuter carriers) is also grounds for being entitled to a full refund. It's a DOT rule.
As the OP identified a change from a 747 to a 4-class 777, I'm assuming these are BA flights as AA does not operate the 747 and BA 777s are 4-classes (F, J, PE, Y) while AA 77s are either F/J/Y (don't think MCE is considered a 4th class) or J/Y.