Well, downgrading is as easy (
) as jailbreaking (over the serial port if necessary) and then accessing a linux terminal shell via the serial port or an SSH-enabled diags boot partition, from which to overwrite the rootfs via `dd`. And if you don't know what that means, you probably shouldn't risk trying.
You can find the old firmware here:
Tutorial FGA - Frequently Given Answers post #7, in the references section.
Just unpack it with
KindleTool.