Hi podnot,
You exchange the header & mainav position by creating a new layout & assign it to a menu page. Here is my guide:
1. Create a clone layout of Default layout by making a new .xml file called: default-clone.xml (This file have same content like default.xml) in path templatesja_ziteetclayouts .
In this new xml file, exchange line:
<block name="header" type="header"></block>
<block name="mainnav" type="mainnav" main-inner="1"></block>
2. Create a new profile in Template Manager >> JA_Zite_Default >> Profile, which you choose the clone layout in Layout Settings tab.
3. Then assign new profile to your chosen menu page in Menu Assignments.
Save. Clean JA T3 cache.
Leo