No, purchasing the Season Pass will not reset your challenge progress. Your completed challenges and current position on the reward ladder remain exactly where they were.

What happens when you upgrade:

• All previously completed challenges stay completed
• Your current progress and points are preserved
• You can continue from exactly where you left off

Pro tip: There's no penalty for waiting to upgrade - you won't lose any progress. This makes it safe to try the free version first before deciding to upgrade.