Yes, it’s safe to rename WordPress built-in roles.
Role has 2 attributes: ID and name. WordPress and plugins use role ID internally, but shows to a user role name. When you rename role – User Role Editor changes the role name only, but ID will stay the same. So when you rename ‘Administrator’ to ‘Power user’, renamed role still will have the id ‘administrator’. You will see in the URE’s drop-down list ‘Power user (administrator)’, for example.