Originally Posted by LarryJ

We don't see those kind of details on the weight and balance calculations. We see the limiting weights and which limit is the most restrictive. Everything else is handled by a Load Planner in Chicago.

They start out with the planned information for bookings, cargo, assumed checked bags, and weather. As departure time approaches, the estimates are replaced with actual data from actual bags checked and passengers and cargo boarded. The initial load plan, for the rampers, is based on the estimated data and is also refined as estimations are replaced by actual data. Sometimes they will have the rampers move bags from one pit to another when the actual data differs significantly from plan. Other times a pit can bulk out before the planned weight for that pit can be loaded and they can't load anymore into it.

The Load Planner in Chicago has no reason to restrict upgrades unless he needs to do so to prevent the C.G. from being too far forward.

As far as how much difference moving three passengers would make, there is a hard line at each limitation. If the calculation returns a C.G. that is exactly on the forward limit then you are good. If the calculation comes up 0.01* over the limit then you are not and something has to be moved. There is no grey area were it is okay to go "just slightly" out of the limit.

(*C.G. is presented in percentage of Mean Aerodynamic Chord (%MAC) and is calculated to two decimal places (at least on the Boeings I've flown)