I'm a bit confused
TBH it sounds to me a little bit like you're "trying to run before you can walk"....but I'll give I'll it go.
Use Tokyo NRT (RJAA) to JFK (KJFK) as an example (rather than HK, simply avoid the north pole..) if you want to fly the shortest route, a great circle track, you'll probably start off heading north east, as you progress the heading will become east, then in the later stages of the flight south easterly, so yes, the required heading is changing throughout the flight.
Great Circle Mapper
I hesitate to go into specialist navigator territory but I think in the "old" days of plotting charts and plotting routes by hand the way you would fly this is by breaking the very long route down on your chart into a series of shorter sections (e.g every crossing of intervals of 1 or 2 or 5 degrees of longitude) measuring the initial heading for each section of the chart, (true heading) and then derive the magnetic heading (from the chart/tables) and writing it down so that you knew what heading to fly on your magnetic compass.... If you then went off and actually flew the whole thing, steering the aircraft using the heading selector
you would indeed have to adjust your heading as you progressed along the route - every time you'd established you'd flown another of your 1/2/5 degrees of longitude defined sections as used in planning you'd have to tweak the heading knob to required number you'd written down.
Nowadays we generally don't do that.....an aircraft flying a route "coupled" to GPS and/or Inertial navigation should in theory follow the great circle route between two consecutive loaded waypoints (e.g RJAA to KJFK) as if on rails, without any pilot steering inputs from the flight deck. What we do do is monitor the heading changes en-route to ensure correct navigation, rather than inputting the heading change ourselves. Whether we are looking at a magnetic or true heading depends mainly on pilot selection, though it's usually True heading at High latitudes.
I hasten to add in the real world we'd probably not be flying a 5000 mile plus direct great circle route, due to airspace restriction, weather, etc....
Hope that makes sense...to someone....anyone
Edit to add for the purists: Given the context I'm invoking the KISS principle, ignoring wind, assuming heading=track)