How can I transfer weights from one bone to another? So I have a model in Blender which I would like to port to the source engine, in order to do this however, I have to change the skeleton to one that will work for the source engine. 

The trouble comes when I switch the rigs I loose all the weight painting. I need a way to copy weights from one bone from one rig, to another bone in a different rig.

Any help is greatly appreciated.
If the two rigs has the same bone names, then you can transfer weights by selecting your mesh and going into the modifiers properties tab and set the Armature modifier to the new rig instead of the old one, then modify the parent of the mesh to the new rig as shown below: