I can confirm Windows update download continues in multiple sessions. 2 days back when I started 8.1 update from Windows store, it managed to download 38% (of 3.6 GB). Yesterday when I started my laptop and checked for 8.1 update, it continued from 38%. Finally sometime after 2 AM, I saw notification to restart Laptop. After restart, some installation happened, and Laptop booted in 8.1 🙂