Changing root view controller of a iOS Window

Is the root view controller of a iOS Window usually initialized once in the beginning to a tab bar controller or navigation controller? Is it okay to change the root view controller multiple times within an app?

I have a scenario where the top view is different based on user action. I was thinking of having a navigation controller with the top view controller having the image of the splash screen, and pushing/popping view controllers as required. Alternately, I can keep changing the window's top view controller. Which will be a better approach?

It is more usual to use a "presented view controller" (presentViewController:animated:completion:). You can have as many of these as you like, effectively appearing in front of (and basically replacing) the root view controller. There doesn't have to be any animation if you don't want, or there can be. You can dismiss the presented view controller to go back to the original root view controller, but you don't have to; the presented view controller can just be there forever if you like.

Here's the section on presented view controllers from my book:

http://www.apeth.com/iOSBook/ch19.html#_presented_view_controller

In this diagram (from earlier in that chapter), a presented view controller has completely taken over the app interface; the root view controller and its subviews are no longer in the interface. The root view controller still exists, but this is lightweight and doesn't matter.