With the enhancement of the MH website I am now able to modify my seat assignments. In the past if I didn’t select seats at the time of reservation you had to call.

From the desktop version main MH page click Manage, then when your reservation comes up, scroll down to service details, click “additional services” then a page opens where you can click either seats or meals. Clicking seats brings up the seat maps which you can edit - at least I could on upcoming October business class flights HKT/KUL/KTM and return. After making the change I got an automatic email with the new seats listed on my reservation.

Of course the IOS app doesn't work properly and can't even find my reservation.