At my company, we are transitioning (fully) from ClearCase UCM to Git. We will not be using Clearcase after some time using Git.
In the transition, are there any go
At least, with ClearCase UCM, you have full baselines to work with, that is labels which are set on all the files of a given component.
By experience, you never import the full history from ClearCase to a Version Control Tool which is based on revision across the the referential (see the difference between ClearCase and Git: ClearCase has an history set file by file, not "revision by revision")
What I usually do, as I mention in my answer "Sync GIT and ClearCase" is:
Try this tool to import history from ClearCase to git: https://github.com/charleso/git-cc
Regarding the reorganization of repos when migrating from ClearCase to git, there were few discussions on SO, for example this one: Sync GIT and ClearCase
I would not retire clear case immediately. What you want to do is commit the tips of any branches in clear case and any tags. This should be done in a fashion that mirrors what happened in clear case (preserve logical branch points, etc). Use git from that point on. Refer to clear case if you need to dig into history. Eventually archive clear case and only restore it if you are in a bind to see some old commits. Importing old history is not worth the trouble in my experience. It is often very time consuming and error prone.