Actually, sorry, I missed part of your post. The post below about render layers may still be helpful, so I’ll leave it, but if you want to do an entire animation and then go back and change parts, then you should render the render layers out in full animations (you can turn on the option to render only a selected render layer at a time in the Render Layers dropdown) and then composite them in the Sequence editor, which has much less powerful compositing options but will do the basic job of alpha-over and stuff like that.
Now on to the original post, in case it’s of use:
Separate your king and your pawn to separate layers (Layer 1 and Layer 2). Go to Render Layers in the render buttons and associate Render Layer 1 with Layer 1, and add a new Render Layer, Render Layer 2, and associate that one with Layer 2 (this should be self explanatory when you see how the buttons are laid out. The upper button set is actual layers, just as a visual aid, and the lower button set is the layers that correspond to that Render Layer.
Then open up a Node Editor window, Select the render button (little picture button) and click Use Nodes. A Render Layer input node and a Composite output node will appear. The input node will be Render Layer 1 by default. Press the space bar and add a new input Render Layer node, and select Layer 2. Now you’ve got two input nodes. Press space bar again and add whatever node you want to use to combine them. For example, Color > Alpha Over. That will let you put one above the other (although, in the node input, the top image is actually plugged into the bottom plug, so heads up on this). Output from that should go into the Composite Node.
You will probably also want to add an output Viewer node, which will allow you to view the output of any node. You can view this output (after you’ve rendered the render layers once) in the UV/Image editor.
Cool things > each Render Layer has a selection available of what it renders. You can toggle halos, edges, etc on and off. So you could render your edge only on one render layer and use that as an input in the nodes system, or whatever. Also, once you’ve rendered your render layers, you can futz around with the nodes all you like and your final rendered output will change accordingly.
When you render with nodes, be sure to select “Do Composite” in the render buttons.