The question also arises as to whether the OP was ever offered a choice: go economy now or business class later when seat is available.

When my house guest faced a downgrade on a recent PER-BNE-CNS she was not offered a choice and lied to about holes in NE being full (see another thread - no need to repeat details herein).

IMHO very very bad form by QF and very worrying that the airline appears to be making the practice commonplace.