I migrated all my servers from DTC-PANEL to ISPconfig. DTC wasn't upgrading to Debian 7 fast enough, so I migrated to an ISPCONFIG setup.

It's not something you wish to do in a production environment, especially without a second server. If you are planning to do this in a production environment, I recommend setting up a second server and migrating one client at a time.

This will make it easy for both you and the client, you can schedule the downtime with them, and off you go.

Once you've migrated all your clients over I recommend you turn off the first server and wait 24-48 hours to make sure that you did not forget something important. If you've had your first server off for 48 hours without everything going haywire, then you are ok to proceed.

I recommend formatting the first server and making it into an ISPCONFIG secondary WEB, DNS and mail server.

This will be helpful at the time of any outage. It be DNS to the primary, and queue mail for the primary while it's unavailable.