As suggested by Amit, both will work.
Recently I did the upgrade of BI 7.0 to BW 7.3. The original BI7.0 the objects were in 3x flow only.
Upgrade went well, we suggested the migration to 7x flow as SAP has stopped supporting 3x flow from April 2013.
So Yes the data flow will work, but its advisable to migrate.
With BW 7.3 migration toll is also available i.e. TCode RSMIGRATE.
This toll wont take care of Hierarchy flow, i have created/posted a doc for same.
Thank-You.
Regards,
VB