MH - in blatant contrast to all other airlines I know - has unfortunately chosen to collect fare differences for rebookings, even though they cancelled the flights. Complaining to MH is indeed useless as this is their policy.
Since this was for a TP run and you appear to be flexible, I`d just wait for lower fares to pop up between now and December 31 (as that is the deadline for free rebookings) or just get a refund and rebook somewhere else.
If you booked the ticket in a region with more customer-friendly legislation (say, in the UK) and really want to challenge this policy, please go after them in court...
https://www.malaysiaairlines.com/hk/...-schedule.html