The entry says any Lich, not A Lich. I would argue that a Dry Lich is a variant of a Lich, and thus a Lich itself. By my reading, yes: A Dry Lich can become a Demilich.SRD“Demilich” is a template that can be added to any lich. It uses all the lich’s statistics and special abilities except as noted here. A demilich’s form is concentrated into a single portion of its original body, usually its skull. Part of the process of becoming a demilich includes the incorporation of costly gems into the retained body part; see Creating Soul Gems, below.