Ever since I started to use the SFM I've wanted a simple way to swap the heads of characters without completely decompiling the model in blender and swapping them the hard way.

This is a relatively quick way to do it, but there are issues that can arise, most notably related to sequenced animations and rigs

Simple enough. Spawn your two subjects. In this case I picked the Pyro and the Spy. My objective is to remove the Spy's head and replace it with the Pyro's.

To do this right click on the specific head bone of one of the characters and click "Add scale control to transforms" Next. Shrink the character's head until it is gone. Do this by clicking on the bone's "_scale" value and setting it to "0.001". It's recommended to also shrink the neck bone but some models might not look good upon doing this.

This is the hardest step. And the one you might require experimenting with.

First. Add scale controls to the other character's head and the character's root transform
Next. Right click on the slider for the head and click "Remap Slider Range"
This is inorder to increase how much you can resize the character's head. Since the default 10 is quite low. I recommend setting this value to 100.









Press okay and now click rootTransform_scale and shrink the other character's body. What is most important about this step is
I cannot stress this enough but moving the slider all the way to the left will just shrink the model until it disappears and will not allow you to increase the size of it's head. If you do not set it to exactly the same size as the Spy's head model, you will not be able to lock the Pyro's head to the Spy's body.

In this instance. Double click the slider and type in the value "0.001"
As you can see. The Pyro is now the size of an ant. You might ask "Why would we want that?" Well, resize the character's head. Move the slider all the way toward the right.

Note that this all depends on the size of the character's head. If you have found the character's head is still too small, increase the size of the slider range from 100 to something larger like 150

Move using the Root Transform bone. Or by clicking it's entire model in the animation set editor. And place it on the new character's neck.
Now lock the head to the new body by dragging the first character's head bone ontop of the second's root transform. Just as you would lock a hat to a character's head or a weapon in his hands.

If done correctly, when you try rotating the Spy's head bone, the Pyro's head will rotate instead. Now you may use sequenced animations as you please with the Spy's model.

Naturally this method is nowhere near perfect. It does have flaws.

This is best done for still images. However you might notice once you apply a rig to a character the new head disappears
This is because the rig will reset the new head's scale. You may either resize the head, or equip the rig on the character prior to locking the head to it's new body.

The other issues that might arise mostly fall square on not giving the first character's head the same scale as the root transform of the second character. Or attempting to lock the first character's head bone to the second character's head bone. This can cause numerous problems and it really isn't worth the headache.

Practice with this technique a little. It's something I've only just discovered using rough experimentation. Every head may require a unique scale so be sure to judge the slider range accordingly and the head size.