It seems unlikely that the scenario OP describes after the equipment change was the desired result for Delta, let alone the passengers. Re-accommodating after an equipment change to a plane with a extra cabin happens infrequently enough that either the automation of it is not perfect or the manual actions required of the staff might lead to procedure failures.

Since OP was in the end upgraded, it is clear from those facts that errors were made in determining FC upgrades. While it is true that the DL gate staff has final decision making authority, that doesn't mean they are doing it according to the proper internal procedures. OP was asking for DL to apply the policies known both internally and publicly and in the end that happened. To say that OP had no cause to ask for this is....wrong.