Hi Nando,
Generally speaking you will need some down-time or posting-free period in order to guarantee data integrity. There are certainly ways to minimize downtime.
Please see SAP Note 753654 - How can downtime be reduced for setup table update. You can dramatically reduce the downtime to distinguish between closed and open periods. The closed periods can be updated retrospectively.
Last but not least, you might want to have a look at the following SAP Notes:
- SAP Note 436393 - Performance improvement for filling the setup tables;
- SAP Note 602260 - Procedure for reconstructing data for BW.
Best regards,
Sander