Hi,
Let me try to answer your questions.
Q1. Will BW transports work from Global/RDBMS to Regional/HANA ? Does Transport work on different SP levels of BW?
Q1. It might work but this approach is certainly not recommended and adviced. The instances which are part of the transport landscape should be equal with respect to release, SP level, database and operating system (for the OS and DB also release and patch/SP level).
Q2. What is the SP level of BW-7.4 where we can avail full functionality of HANA?
Q2. SP09; please also refer to SAP BW 7.4 SP9 powered by SAP HANA and Roadmap.
Q3. What are the changes necessary to do to the Global codebase to enable Regional to convert its instance to HANA?
A3. BW and ABAP functionality will be compatible. Only HANA will enable new scenarios and programming approaches which were not possible before. I don't think that you have to or you are able to do a lot in this area.
Best regards,
Sander