Currently, there are no plans to allow on-demand rerolling of Daily or Weekly Challenges, regardless of whether or not they've been completed. New Challenges will populate in your Vault Cards every day at 9:00 AM PT / 12:00 PM ET / 5:00 PM UTC for Daily Challenges, and Thursdays at 9:00 AM PT / 12:00 PM ET / 5:00 PM UTC for Weekly Challenges.