-From its DEFAULT position,export the new head as an obj.

-Open a new scene, import the head obj. Don't move it!

-Open the target figure

-Make the head-to-be-replaced invisible.

-Move the BODY of the target figure to align it with the new head. Again, don't move the new Head from its default position - just move the figure's Body.

-When they align, do the Replace Body Part With Prop thing for the heads and then deal with either realigning or replacing the eyes, and re-importing the head & eye MTs.