The Migration Preparation Tool (Source Tool) was updated when SBS 2008 with SP2 was released. If your SBS 2008 media includes SP2, make sure you're using correct version of the tool provided in SBS 2008 DVD 1.
This new tool now has the ability to check for health issues that could cause a migration failure, in order to get the rules, you need to select "Download and Install Updates" when prompted. Once updated, MPT will check for a number of conditions, one being the source server in Journal Wrap. In order to determine if your source server is in Journal Wrap, the tool will scan your File Replication Service event log looking for Event Id 13568.