andrasz If that is the case then I cannot see how this can happen - integration suggests the sharing of the same seat plan within or across systems; even in disparate systems integration is continuous or regular communication; not electronic transfer of data one-time at flight close. This is also the essential dispute in the report. Further illustrated in the report by the outline of W&B being completed on an Excel spreadsheet - unless this is for illustrative purposes only. However that would then raise the questions to why the report does not show the actual documents or simulated screen shots of the condition.
If as you suggest it is fully integrated, then this suggests both parties (passenger services & load control) had the same seat map (320) in the solution up to and including the time load control made the change to 321 from 320. In an integrated solution that process would fail if occupied seats on the original (320) seat map did not exist on the new seat map - to simply enforce a positive reseating of mismatched passengers [seats] and new boarding passes issued. If all seats did map across 320/321 the process would execute without error and load control would know exactly the seating condition on the new aircraft (321) and the resulting out of trim situation.
This is now the most interesting and critical part - if this were the case, which would be the case in an integrated solution; with little 'belly load' to play with, the load controller is left with only being able to re-arrange seating to correct balance now that they are aware of the out of trim condition. That would mean (with high probability) two things then happened.
(1) positive action to change from seat row trim to that of cabin area,
(2) positive action to overwrite the actual cabin area distribution with a new (now required) seating distribution to result in a safe balance condition.
Now - if that is really what happened and those direct and positive actions, taken in and with full knowledge, to correct an unsafe condition were then failed to be immediately communicated to those who would be responsible to execute the physical change in seating to insure the change was effected on the aircraft and not just simply realised within the computer system and on the final load sheet document, well .... honestly, it's not simple, its incredibly negligent.