Copying skin weights is an important aspect of rigging that allows you to make significant changes to your mesh without having to repaint weights from scratch...a massive time saver.
0:00:00 Overview and setup
0:02:18 Step 1 - Skinning fixed/updated mesh
0:03:10 Step 2 - Copying Weights
0:05:46 Step 3 - Cleanup (and final notes)