As I understand it, accounts on the old server (oldcossacks for this post), are being suspended on oldcossacks. This is most likely why you can't get into cPanel. Suspending the accounts while they transfer is a precaution we are taking because in many times before this, users have made changes to files or their accounts on (oldcossacks), after they were transfered. DNS hadn't caught up with the change, so their changes were ONLY on their old accounts, not the new server, which caused many headaches for a lot of people.
As the status update shows, about 1/3 of the accounts have been moved. Just keep watching.
You don't have to do anything to make sure you don't lost data during this move, however I do recommend making backups, maybe bi-weekly, or monthly, so that you have a backup of your data should any problems occur with the servers that are beyond our control.