Turning a phrase negative (e.g. good → not good, and a question → not a question) is called negation. One can say "I negated the phrase".
What is the act of turning a phrase into its opposite (e.g. good → bad and a question → an answer) called? I'd like to say "I ×'d the phrase."
A gerund sounds forced. "Oppositeing"? "Oppositising"? "Oppositeifying?" Maybe I'm getting too fixed on conjugating "opposite".