The two antonyms for the phrase "alter conviction" would be "maintain conviction" and "confirm conviction." As opposed to altering one's beliefs or opinion, to maintain or confirm one's conviction is to uphold the same stance, despite any new information or arguments presented. These antonyms imply a steadfastness and unwavering nature in one's belief system, albeit with the potential for evolution over time. In certain situations, maintaining or confirming one's conviction may be seen as a positive attribute, as it shows determination and commitment, yet in others it may be viewed as close-mindedness and stubbornness.