Quick answer
Your coverage stays at whatever value was scheduled, not the new market value, until the policy is updated. If a piece appreciates significantly and isn’t reappraised, you’re effectively underinsured on exactly the asset that’s now worth protecting the most, a gap that only becomes visible if a loss happens.
This is why we treat reappraisal as an ongoing habit rather than a one-time task, particularly for categories prone to real appreciation like fine art and collector vehicles.