Dropping a new engine into a 20 year old car is not something that happened everyday.
As we speak, the engine now is on a treiler, on the way to Penang. Will reach there today for the replacement work. If MH operates only 1 single B734, then they might have some problem dealing with this kind of situation. Just like when SQ A380 just arrived. They were new. Little problem surfaced and the planes were grounded. It's not the age. It's the support. Unless you ask MAS engineering to change engine of DC3 or Concorde, then maybe they will have lots of problem.