A few years back when I used to travel LHR - KUL as a student, MH staff at LHR often asked if I was willing to go on an earlier flight. So no harm asking at both LHR and KUL as staff tend to be more "accommodating" than other airlines, especially at current low load factors.

Otherwise 7-9 hours is a pain to spend at KLIA. Suggest you board KLIA Express and in 35 minutes, you're in the city... spend a few hours at MidValley / The Gardens malls with great value shopping; have a meal or two; then back to KLIA for your onward flight.