Hi Debby
It is not mandatory to migrate dataflows before the upgrade
Extract from Note 1686378 (Applicable in case your moving to HANA after Upgrade)
After the migration, 3.x data flows are fully executable; however, so that the performance advantages of the SAP HANA DB can be fully utilized, we recommend that you convert all InfoCubes to SAP HANA-optimized InfoCubes. In addition, we highly recommend that you migrate all 3.x data flows to 7.x data flows because enhancements always optimize regarding 7.x data flows.
SAP provide a migration tool but only in BW 7.4
Thanks
Abhishek Shanbhogue