I think Ive seen it done, so Im wondering, if im right, how? How would you exchangea units head, like give jango, maces head? Is hex editing involved? Or is it something to do with the odf, or is it modeling?

That would be modeling, then re-rigging it.

I hexedited the head off of Dooku. I haven't tried switching it with someone else's but it might be possible.

Someone(or I suppose I could do it...) should try finding another model(besides Dooku's) with the head as a separate MODL chunk and then hex-switch the heads.

It might not work, but it would be worth a shot.

Does that dookuhead MODL chunk define what tga it should use though?

you can remove a head with hex editting, but you can't switch heads with hexing. Like Taiyvx said, that has to do with switching things around in XSI to make a new model.

Actually, you could swap heads with hexediting, it would just be a very sloppy job. Like Saitek mentioned, you can hexedit the head "off" (which is really just making the head invisible). You can do the reverse with another model - make the whole body invisible. Then you can hex the head-body's MODL chunk (heads don't have separate MODL chunks on most models) onto the "headless" model.

Doing it in XSI would probably be better, though.