When you move from SAP7 to SAP7F with the migration tool (where we currently have customers running on the initiative), one of the prerequisites is that you do not have data in your new customized SAP7F.
If you have any structural changes, you cannot transport the new SAP7F to production when there is already data.
A recommendation would be.
1) Move your configuration with the tools from SAP from ZSAP7 to a fresh copy of SAP7F (ZSAP7F)
2) Test with dummy data and transport to production
3) Go live with other business areas
4) Move data for the business areas still in ZSAP7, and note that it's an example only to show an end-to-end process.