Doing a good job in step 2B is the key, so that you can make as many assumptions as possible in 2C.
I also have a tool to simulate merging so that you can see the impact and know which roles are most impacted (compared to what they had before hand).
It is most effiecient to do all that work in the DEV system after it is upgraded and try to pass as little as possible on to testing changes.
Cheers,
Julius
PS: note that if the roles are build manually before hand, then you cannot leverage SU24. So SU25 is just an irritating popup in PFCG and the consultants who implemented the roled are long gone...