The assumption here is that the preparations for migration went smoothly or all issues that were raised have been fully resolved. Since the migration process builds on previous steps, if something has failed it needs to be resolved otherwise later steps will fail too. It is also vital that your server is fully patched as per the Windows Update and the Best Practices Analyzer tool and that all the pre-requisites as described in Chapter 3 have been met.
I'm also assuming that the installation, other than needing updates, went smoothly as described in Chapter 4. If not, you should re-run the installation rather than trying to continue when the foundations are flawed.
There are a number of checks in the migration process, for which if they throw up an error the recommendation is to stop and fix the problem. If the problem is particularly severe or points to a flaw in the original SBS 2003 system, then the most likely way to avoid further issues in the migration would be to follow this...