The changelog is via the message on the Band 2 itself:
Track your heart rate zones on Band: Improve your workout health with notifications for heart rate zones. When you're at 80% maximum heart rate, the heart rate on Band turns orange. At 90%, it turns red.
It's not yet clear whether this is just when working out in the gym or when running or cycling - I suspect it's all of the above. If you're able to trigger this then maybe you can comment? Actually, I'd like to see Microsoft go further, with more 'zones' for the (ahem) older users, perhaps green for at 70% max heart rate.