Are you talking about replacing the head of the speaker with another head, or similar? If so, they what you are talking about is Keying, done with a Matte. Also, PrE has Video Merge, in the later versions, and it might be helpful for what you want.
If the replacement "head" was shot against a green screen, then things get a whole lot easier. This ARTICLE, while for other programs, might be useful for some ideas.
Depending on exactly what you are shooting for, it might be that another program, like Adobe AfterEffects, will be required.
Can you post screen-caps of your "original footage," and then show us what you would like to replace the "head" with? That might tell us the best method for you to explore. Different footage, and different uses will dictate different methods and maybe programs.
PS - if you do not need exact Keying, you might want to explore Garbage Matte on the Clip in the higher Video Track, as it will be much more precise than the general 4-sided Crop Effect.
Now, this is getting way out of the realm of PrE, but here is a TUTORIAL on a new feature in AfterEffects CS5 (on sale soon), that will greatly speed up the rotoscoping which could be used to superimpose one face over another.
You may also be talking about doing a Picture-in-Picture effect or a Chroma Key effect (in which you place someone in front of a new background).
If you can show us an example of the effect you're trying to achieve, we can likely tell you where to go to find out how to do it.
Or, if you'd like a manual that takes you step-by-step in creating these and other great special effects, check out my new book!