You have right question, we do have discussed about old cases and we thought we will delete the cases if there are less.
else we have to find a way how we can migrate the cases as per new design (Not found any concrete approach as of now)
Yes all users will have access to new application (Redesigned application, access group etc) once we go live with new design.
1. If i start renaming class (Using wizard), it renames classes and rules with new names, then i will not have old rules in development, But if i deploy to next environment (Test, Prod) it will have 2 rules each (old and new) this causes in-sych b/w dev and production.
2. What i know with my exp, is deleting / refactoring code in production directly is not a good idea.