Hmm. A few Worf foreheads from DS9....
"Way of the Warrior":
"Apocalypse Rising", before:
"Apocalypse Rising", after:
The ridges are the same, but the beard is gone and the hair is worn differently. Perhaps the heroes felt that it would prove advantageous at some point for Worf to reveal himself and appeal to the Klingons? That wouldn't be easy if Worf wore a false face.