Personally, I wouldn't make work for myself until I knew I had a problem.
Provided the current position works fine for ducting then I would use the existing hole. So how much further away is the far side of the room opposite the fire?
Once the new system is working, you can check to see what happens when you heat the room. Is the room heating evenly? Is there a dead spot due to the positioning of the inlet vent?
If it's a wood burner you should be okay. My experience with having them at the opposite end of the room is it really only makes a difference with things like heat pumps (though if your house is insulated enough this shouldn't matter too much provided it can catch up and heat the room it's in as well)
callumjohn: but I understand the recommendation is to have the inlet on the opposite side of the room.
I imagine that is to allow heat from the fire to warm the main room, If you have the inlet too close the risk is the airflow in the room is all directed towards the fire rather than away from it....
BUT. If you have no issues with the main room remaining warm now, don't make trouble for yourself...