Removal of Custom Roles (including dynamic) from user profiles. Backend?

We are separating our CS environment into CS and HR (DB split).

Once we have two instances, we want to separate the roles and permission lists from respective users.

Since the no.of users are in thousands, we cannot do it from front end.

Can you tell us a way to do this for bulk users.

Is there any script or utility to do this.

Is removal of rows from PSROLEUSER tables recommended approach?  If yes then, are there any other tables that needs to be updated with the above tables?

Majority of roles are custom and very few delivered roles are used.