HRF implements a subset of BRFplus functionality. HRF compiles the rules into HANA SQL, whereas BRFplus compiles ABAP code. Usually you use HRF for analytical rules whereas BRFplus is used of operational/transactional rules. You use BRFplus in S4H also because of user integration with applications, user management, authorization, life-cycle management etc.
We are currently working on the merge of the two into one solution. There is no migration or so required. Instead you can expect BRFplus to have some more features based on the merge with HRF. However, there may be a need for additional licenses (HANA, DSM).