PW2 serial number starts with 9017. Shipped with Firmware 4.3.2.1 on it.
In terms of how to go about upgrading or downgrading, I grabbed the firmware from:
http://david.smidovi.eu/kindle/firmware-updates/
with the exception of 5.4.5 which I grabbed from Amazon.
Upgrading was done straightforward. Copy the .bin file into the root direction, unplug the kindle, then go menu -> settings -> menu -> update your kindle
Downgrading was done following the Downgrader from NiLuJe's snapshots:
So extracted kindle-downgrader-0.1.N-r10677.tar.xz then copied the .bin file into the root directory, as well as the 'official_update' folder. I added whichever firmware .bin file I was downgrading to into the official_update folder.
Then again, uplugged the kindle and went menu -> settings -> menu -> upgrade your kindle.
After which, I'd reinstalled MKK, again from snapshots (as non snapshots left UYK grey'd out). And again, add the .bin file to root, unplug kindle, menu -> settings -> menu -> update your kindle. Then plugged the kindle back in (after it rebooted) and deleted and readded CM.
I am currently on 5.4.3 as the last one I tried, that had the same 'cannot launch, try again later' error.