A couple of thoughts ..
(a) This being the case, the late reallocation of an A320 load to an A321 would have resulted in Zone D being empty.
Unless a trimsheet be designed to cover multiple models (and that may be both reasonable and feasible for some aircraft) a change in model surely requires a reworked loadsheet ? One presumes an adequate level of competence in those charged with the execution of load control. We have checks and balances and multicrew operations to provide for a high probability of success, providing everyone does a reasonable job. In the simplistic view, the crewmember signing off the completed loadsheet ought to be able to detect such gross errors.
(b) I see some comments regarding the non-intuitive nature of loadsheets. This is especially so with the typical electronic summaries we have seen for many years, now. However, there are various simple ways to get around Murphy's antics ...
I was most impressed with a very simple system we came up with to defeat a lot of these sorts of problems which was implemented for IPEC freight operations in the 80s.
The freight shed did their thing using a normal paper trimsheet - but my comment would be equally pertinent regardless of whatever system might have been used - even an electronic ACARS style sheet such as I was routinely familiar with flying for another operator.
The critical thing was that the IPEC crew then ran an independent check using the load summary to check the trim with a whizz wheel trimsheet. One pilot called the loads in sequence and summed the totals as an independent check. The other pilot ran the whizz wheel to check the trim and the whole exercise took no longer than the time it took the first pilot to call the numbers at a measured pace. In addition, as was typical for freight operations then, one pilot physically ran a can check on weights to detect any gross misloading sequence errors. This is entirely analogous to the cabin senior's running an eye over the cabin and (ideally) then running a quick brief with the cockpit to make sure that the reality matched the storybook to a reasonable degree.
Really, folks, there is no reason, or excuse for, why the load control answers can't be reliably accurate and consistently so.
In the same vein, there is no valid reason why operator training should not ensure that the flightcrew is alert to gross speed schedule it-doesn't-look-quite-right calls. I have no background playing with Airbus but, in another age, we would have picked up alarm bells (on Mr Boeing's products) such as speed schedules encroaching on Vmca/Vmcg regions of the envelope without any undue problem or delay.