Obviously uberSVN is still a very new product so I imagine experiences might be difficult to come by. You're not the first to ask this question though so I might be wrong about that!
The way uberSVN is designed, when you add repositories and choose the 'Import Existing Repository' option, the repository will be copied from the current repository location into the storage location you choose for uberSVN at installation time. That way we don't interfere with your existing VisualSVN installation and you can easily test the migration without risking breaking anything in your existing setup. When you are happy it works you can gracefully migrate from one to the other with very minimal downtime for users.
The one area that might take further thought is how you handle user account creation and authentication in uberSVN. If you can advise how many Subversion users you have now and how your authentication is currently handled I can provide a bit more info on the best way to do this. We have discussed providing some specific migration tools from other systems and it would be very interesting to know how you get on with this. Of course if you need any help feel free to update your question or to contact me directly.
uberSVN Product Manager, WANdisco Inc.