I flashed my n950 to PR1.3 (I guess it's leaked) 10 minuted ago. seems to work fine (till now...)
It is possible to flash from a recent version to an older one, it up to you to decide which version you want to flash to (unless you really messed up and bricked your device in a really bad way)
Anyway, you should be really careful about flashing.

BTW, does someone know how to connect the n950 (n9) to the internet via usb and my pc's internet connection (running linux an win7) without the sdk on the pc like n900 pc-connectivity?