So currently, there is no XML to update a schedule policy for Aux Copies. if you look in books online, all of the schedule policy docs are primarily for "Data Protection" XMLs.
In order to Update a Schedule Policy for "Aux Copy" type, you'll need to use Rest APIs with a PUT method. So Really what your doing, is rewriting the entire Schedule Policy with your Update.
So for this to work,
1)you first need to run a GET call to maintain the Current schedule policy,
3)Finally make another rest call with the PUT method and the new Schedule policy you created.
In our case, we've been deploying multiple sites globally, new Datacenters built from the ground up. SO I needed an automatic way to Create the Sitess, from the primary backup all the way to scheduling the Aux Copy. So far, works like a charm, but a tad bit of a headache tweaking the codes and learning the http client utils.
All this could be easily sovled if CV devs created an XML for APP_updateAUXCopySchedulePolicy, but they're a little bit busy right now.