It will preserve the jailbreak, and you are able to downgrade with official bins through a convoluted manual hack even though the jailbreak installer doesn't work.
Since you are already jailbroken, don't worry.
You can download updates from here:
http://david.smidovi.eu/kindle/firmware-updates/
http://ixtab.tk/kindle-touch-updates.php -- ixtab doesn't have that one.