The blocks produced by this new software would be rejected by other miners still using the unmodified software, because difficulty(block)<difficulty_enforced_by

The blocks produced by this new software would be rejected by other miners still using the unmodified software, because difficulty(block)<difficulty_enforced_by_bomb_at-that_time. Hence: the very definition of a fork, that is some blocks seen as valid by some faction and seen as invalid by others.