My update, Expedia UK contacted the airline and provided the rerouted LH flight via FRA, LH 507 and LH 9762. But the airline changed the class of GRU-FRA into eco(Q) and remained FRA-SIN in Business (C). No extra charges for this option, but will have some charges for any changes afterward.

Quite disappointed that cannot get the C class for all long haul flights, and also frustrate if there will be any changes by the airline in the future.

Another point, It seem like most of us got only refund choices?
Or they provide the unacceptable flights?