Hello Tramano --
I'm moving from S950 to Genos, too.
I decided to start small and scale up -- try one or two registrations and experiment before committing and converting many registrations. The "go slow" approach appears to be a good one because I find that the Genos file/directory organization and workflow is different with respect to the S950. I would suggest learning how to work on the Genos first before starting conversion.
For example, I converted one S950 registration to Tyros 5 registration format using Murray Best's Yamaha Registration Manager tool. This was enough to get me started with learning and experiments.
I also suggest inserting only one USB drive at a time to simplify operations. Registrations store the complete, absolute file path name, including the drive name, e.g., "I:\" Genos uses the path name to find the file and if files have moved, been renamed or reside on a different drive, then Genos will not find them.
Since the S950 has such a small internal memory, I haven't kept very many styles, etc. in it. Thus, I didn't have to think about the internal directory and file structure too much (or at all). Genos has specific directories where it stores styles, voices, registrations, etc. with its internal memory. Working on a single USB drive alone is more flexible and gives you more choice about how to organize your files. Thus, it might make sense to work solely on USB drive for while, learning about the internal memory, and then start setting up files, etc. on that nice big internal memory.
Hope these tips help you out -- pj