Re-render only portion of image? Is it possible to render only a portion of your image while leaving what was previously rendered in the same render slot?

For example, if you rendered an entire image, then wanted to render a small portion of it again while keeping all of the original render except the small portion that would be different?
Not on the same slot. Re-rendering will wipe the information from the current slot. 

To save the previous render use a different slot (You might want to save the image to an EXR file).

To render only a section use the Border Render option (CtrlB), then you'd need to composite the new section on top of the old one using the compositor.