I edited the Model in two different Files... I need to stitch them together

okay, i had the model file… and then i copied that file to do the animations. when things went wonky with the animations, i adjusted the model, specifically the Jaw, Head, and Tongue.

today, i went to fix the mesh and weight paints on the model file. i fixed the Eyelids and Eyes and also re-fixed the Head. but now i really really REALLY need my Tongue and Jaw vertex groups, weight paint, and modeling (because yes, i adjusted the lips. wait. or do i just need the… darn it, i edited the… i’m so dead.)

i THOUGHT i could just copy the weight paint vertex group from the second file’s model. to this end, i appended it to a new layer.
i selected the OLD model, then shift-selected this NEW model, selected it’s jaw vertex group, and use the vertex group V arrow button to “copy group to selected.”

this failed because the geometry was changed.

is there any good idea… how i can merge the two models together to keep the parts i want from each?
if i can recreate the new polygons i made on the head to the head that has the weight paint i want… that will still be fubar, won’t it? ::SIGH::