policy_name and policy_group are like any other non-attribute node property, such as environment. You probably want to set these during your bootstrap process most of the time, but sometimes you need to change them. If you have a small enough infrastructure that using knife makes sense, then you should be able to use knife to modify them. Right now, bootstrapping (with knife bootstrap or other method) works like it should, but modifying nodes with knife node edit doesn't work, but that's only because of a bug (again, knife node edit does not save policy_name and policy_group · Issue #4364 · chef/chef · GitHub). That bug is fixed in master, so the fix will go out with Chef Client 12.7.
As for setting the revision of a particular policy that is applied for a policy group, that is only done by chef push.