The phrase "a world of difference" means a significant change or improvement in something, often in terms of contrast. There are several synonyms for this phrase, including "night and day," "chalk and cheese," "poles apart," "drastically different," "utterly distinct," "completely dissimilar," "markedly contrasting," "radically divergent," "vastly varied," and "fundamentally unlike." Each of these synonyms can convey the same idea of a noticeable contrast between two things and emphasize the transformation that has occurred. Using these synonyms can add variety and depth to your writing, enabling you to convey your message more effectively.