Note that your NRTKUL flight, which I assume is on MH, is a long time from now. Given changes in flight times, and the various changes occurring at MH, it's possible that your currently booked NRTKUL flight time will change. As a result, it then would be easier for you to convince an agent to make a change to that flight. But, they're not likely going to allow you to change it to a later date (therefore creating a stopover). They MIGHT let you change it to a different flight assuming the NRT connection is still less than 24 hours.