Microsoft have updated the SDK to support the new Microsoft Band 2 and released new design guidelines for developers. The experience design guideline for the Band 2 shows off where sensors are location on the new Band and you can see how Microsoft have improved the design in the 2nd gen device.
By comparing the documentation for the original Band and the Band 2 you can see that Microsoft have improved the display making it 320×128 on the new Band which should mean the Band can provide more on-screen information. Microsoft Band 2: 254ppi screen size measures 320px wide by 128px high, Band 1: 243ppi screen size measures 320px wide by 106px high. I also noticed the standard font sizes are 1px bigger on the new device.
The document show a subtle change to the guidelines of the new Band 2.