How to rename DFS shares

I don't think it's possible to rename a DFS Root but there's nothing stopping you from creating a new DFS Root and transitioning the users to the new DFS Root.

You absolutely can populate the root of a DFS namespace with content. There's nothing stopping you and it'll work fine.

In Windows 2008 DFS you can selectively disable root referrals so the problematic issues associated with partial replicas that existed in Windows 2003 DFS are removed.

Background for an unlucky soul who might still be using Windows 2003 DFS and who comes upon this post: In Windows 2003 DFS the main reason for not populating the root of a DFS namespace with content was because you couldn't disable referrals to incomplete replicas. Imagine a scenario in which you create a DFS root, populate it with 100GB of content, create a second root target, and enable replication. Referrals to the new root target, which isn't fully replicated, can't be prevented (unlike referrals to links within the root, which can be disabled on a link-for-link basis).