Unfortunately I believe the only option is to purchase the Author Change Add-On FAQ from Jive Pro Serv. And it looks like that doesn't support bulk actions (i.e. can't reassign ALL content from a given user) which makes it of limited utility.
Here's the idea: Change Author of Content
Other discussions on the topic:
Hey Adam, thanks for the answer.
Unfortunately, It seems like with any "common sense" feature, Jive requires a 3rd party for development. The Change Author of Content idea was created in Jun 30, 2011 and is still in review. It's so frustrating.