"Well this could be expensive
"Well this could be expensive for as Morten pointed out, the remote cable is in most cases is put on the floor and the boat is build around it(it seems this way at times).
So without seeing the boat I can't say just how much work / time would be needed for changing out the remote cables.
I have had some that take an hour and others that took most of the day because of all the tie wraps,cable retainers,seats etc,that were in the way.
The best thing is to have the cables tested.
Unless this is an old boat that has been left out most of it's life the chances of the remote cables being bad are slim.
Just disconnect the cables at the stern and see if the remote operates easy. If the remote moves free then just replace the drive cables.
The drive cables most of the time are the cause of hard shifting.
Remote cables last a long time.
But if this boat has been around salt water,that's a different story."