Migrating Budget Data During the Fiscal Year 
If you intend to migrate during the course of a fiscal year, you should carry out document-based migration.
In this case, data of the first BCS year was previously entered in Former Budgeting and should subsequently be migrated to BCS via documents as they correspond to the current year’s data.
If you want to switch during the course of a fiscal year, y ou should postpone the activation of BCS until the point in time that you are ready for migration and can start working in BCS.
This is a safer way to keep the full functionality in FB, including the data entered for the first BCS year.
Read the corresponding system documentation and then carry out document-based migration .