thanks eddie ill get started on this!
I'd look at Track Matte Keying Effect (in Eddie's tutorials). Depending on how linear the head "movement" is, you can probably do a couple of Export>Still from your new "head" footage, and then bring these into Photoshop. There, create a mask for the head and use that mask as your Track Matte. You can use Motion>Position (and Scale) on it, along with these Effects on your new "head" footage, to mask out eveything but the head for your new composited footage. The less linear your head movement is, the more changes in the Track Matte you are likely to need. AfterEffects would make this a bit easier, but unless you have a lot of sizing changes and movement, you can probably do a nice job in PrPro. Tip, you might want to add a touch of Gaussian Blur to the edges of the Track Matte masks, just to soften the transition to the underlying footage. Tip 2: if you get the size and movement set via Keyframes in either the mask, or the new "head" footage, you can Rt-click and Copy, then Rt-click on the other Clip and choose Paste Attributes. I'd make sure that the Duration of both is the same, so the Keyframing copies over perfectly. Establish your Durations first, then work on the Keyframing.