The most important backup of all
So before you run the source tool on your SBS 2003 box in addition to the normal backup. In addition to the possibly paranoid backup you need to do with a third party program like Storagecraft, Acronis, DriveImageXML (paid not free), make sure you do ONE MORE BACKUP.
One that I'd argue is the MOST important one of all.
Just and nothing else but, the system state backup.
With that you can roll the AD back should something occur. Don't stick it on your normal backup location as well, ensure that you park it several places, on the local drive, possibly on a usb drive but ensure you have a system state backup as it's key here before you being the migration.
Read the complete post at http://msmvps.com/blogs/bradley/archive/2009/11/03/the-most-important-backup-of-all.aspx