The phrase "haddest change of heart" is often used to describe a sudden shift in someone's thoughts or feelings. However, if we're looking for antonyms of the phrase, we might consider words like "steadfast," "determined," or "resolute." These terms suggest a sense of unwavering commitment or conviction, standing in contrast to a change of heart. Other antonyms might include "unwavering," "inflexible," or "unyielding," which all connote a sense of rigidity or immovability. Ultimately, the opposite of a "haddest change of heart" is a strong and unwavering conviction, which cannot be swayed by outside forces.