I just purchased a Raspberry PI 3 and dropped my old SD card with Raspbian OS, which ran flawlessly on Raspberry PI 2, on Raspberry PI 3. It won't load or do anything. So, what version of Raspbian OS can you run on Raspberry PI 3?
Yes, there are some changes (I would say major changes) for the Raspbian OS with regards to the Pi 3, because of new processor, inbuilt Wi-Fi and Bluetooth, new version of scratch, python, new Linux Kernel v4.4 and many more..
For more detailed information on what updates are in the Raspbian OS and how to upgrade, please refer below link. Raspbian update related to Pi 3