How to make sliding doors?

Anything as simple as a sliding card, or a card whose texture gradually becomes transparent, would do.

Question: how do I access an element (such as a card), after it has been reparented to render? (If I would like to change its texture during the game.)

Sorry for a newbie question.

A node is the same node after it is reparented. You can still manipulate the same object in the same way you would before it is reparented.

Example:

mydoor = loader.loadModel("yourdoor.egg")
mydoor.reparentTo(render)
mydoor.setX(1)

It’s basically the same as

mydoor = loader.loadModel("yourdoor.egg")
mydoor.setX(1)
mydoor.reparentTo(render)

You can also obtain your node from the scnee graph (render) from any other place by issuing methods on render. See the manual or the API reference for that.