Starting with Cassandra 3.0, we include support for converting pre
CASSANDRA-7653 user tables, until they will be dropped by the operator. Converting e.g. permissions happens by simply copying all of them from permissions -> role_permissions, until the permissions table has been dropped.
Upgrading to 4.0 will only be possible from 3.0 upwards, so I think it's safe to assume that the new permissions table has already been populated, whether the old table was dropped or not. Therefor I'd suggest to just get rid of the legacy support.