Keep in mind when you migrate the server, there will be no packages assigned to your groups unless you put them there manually. If there are packages already assigned to the groups, they would be old packages and the clients would do nothing at that point since they are already running that build. You would then have to manually assign the RU6 packages in order for your clients to migrate to that build.
If you have certain clients you don't want to migrate. Move them into their own group temorarily, then assign the RU6 package to the original group until they migrate. Once migrated, you can delete the RU6 package from the group and move the clients you didn't want to migrate back into their original group.