Timing of Migration 
The timing of your migration efforts is crucial to deciding what migration path your organization should take.
SAP recommends that you migrate at year-end, because in this scenario only historical data from previous years is migrated. All BCS data for the years when BCS is active is directly created in BCS without any migration effects.
For organizational reasons, you may not be able to migrate at the recommended point in time. In this case, you can opt for a migration during the fiscal year.
You have the following options:
Migrating Budget Data at Fiscal Year End
Migrating Budget Data During the Fiscal Year
Depending on when you want to carry out migration, the migration path will differ.