Is there a way to animate the replacement of one object by another in Keynote?
It is possible to have objects run their animations at the same time. This is done in the Build Order window, which is available from the Animation palette (button at the bottom labeled "Build Order") or via the View &gt; Show Build Order menu item. Set one of the 2 animations to fire normally, either "On Click" or "After Transition". 

Set the second animation to go immediately after the first one, either by setting its build order number to 1 more than the other, or by dragging and dropping in the Build Order window so that it is immediately below the other. Select the second animation, and now you have the option in the Start menu to start "With Build x" where "x" is the build number of the animation directly above it. 

So, by setting one item to have a build in of Cube and another item to have a build out of Cube and setting them to go at the same time, I can get the effect I want of having the one replaced by the other.