[_ck_]

You need a basic understanding of filters vs actions in WordPress before you can get this done.

You can’t replace anything via add_action(‘post_form’… because it’s an action and not a filter. Calling it as a filter doesn’t change the fact that the hook itself is an action.

Filters give you the opportunity to change the data passed.

Actions are just points in the processes that you can inject activity into, but you can’t change what was before or what is coming after. No data is passed to you, no data is taken from your activity.

You cannot change forms this way on the fly, it’s impossible (unless maybe via javascript).

If you want dynamic themes, start with my theme switcher plugin and find some way to keep track of what theme is used without cookies (maybe sessions instead).