I looked at the how to upgrade items and how to upgraded a bonded item FAQ and I get it for the most part, until you get to a Signifier's Mask.
The Mask itself is not a magic item. So it is not covered by named magic item. Also the Hellknight signifier class says it stacks with the class that gives you the bonded item to figure out what it can be upgraded into it. so obviously it is intended to be upgraded in some fashion.
Now if it was a Ring... it could be "upgraded" to function as a ring of protection or as a ring of freedom of movement or any other type of magic ring. But it is still your original bonded ring.
As a Mask which takes up the helm slot; Can it then be upgraded to function as a helm slot item? Such as a circlet of persuasion? Does it work just like the ring, amulet, wand or staff would but as a Helm slot?