How do I maintain users chatter history(posts, followers, feeds,etc) when the user is given is downgraded from salesforce to a chatter license? Normally a new user has to be created when license has to be downgraded and hence all his chatter history gets lost. Any workaround for this?
You'd need to export all of the data and then import it using the id of the new user. There is a Salesforce feature activation that will let you set the createddate of the imported records so that things appear in the timeline in the correct order. I've done this with followers, group membership etc but not for posts/comments, so I can't say for sure that aspect of things will work.
I've never known Salesforce to agree to downgrading a user license - its always required a new user and data fixes. I suspect they don't want to make it an easy option as more people would do it, affecting their license revenues.