Render over the previous render result Is there any way to get Blender to paint a fresh render over the top of the previous render, to make it easier to see the result of changes?

So: I render a scene, then make some changes, then hit render again, and instead of being blank, it shows the previous render result, but gradually replaces it with the new result.
You could technically do this in the compositor, but you would have to make sure to save each of your renders out as images, and then be diligent to add them all back into the node editor. Then you could use (overlay, add, multiply, Mix, etc...) nodes to see the backdrop change. 

It may be a very good way to learn the "Gut Feel" of what your parameters are truly affecting.