I poured over the Jack marshall rules, and didn't find this, so I'm betting on "no."
However, Is there any rules saying that you can't use a jack marshall to take a warjack from a warcaster's battlegroup.
Better question, does a warjack need to be autonomous or inert to be "reassigned"?
Totally had my Nomad's cortex BLOWN off in 1 shot and it ran around without a brain doing NOTHING for the rest of the game until somone ripped his sword arm off. It was lame. I had a jack marshall on the table and thought it'd be nice to move this jack from my warcaster to my jack marshall as it could actually do something then...
Just a thought that begged the question.