Impact Analysis for R3 role migration to S4

Hello all

I´d like to know how different from R3 is the security architecture of S4 in terms of roles/authorizations/objects. I understand that at least 80% of Trx are the same in the new system, so a R3 landscape can be migrated with minimal impact. Special analysis need to be carried out for customized trx. So, we can migrate a R3 role landscape with minimium impact? or do we need to re-design technical roles during S4 implementation?

Appreciated your comments and feedback!