demote (one) from (something)
To reassign one to a role or position with less authority or responsibility. I'm worried that if the boss finds out this blunder was my fault, she'll demote me from team manager.
demote someone from something (to something)
and demote someone (from something) to somethingto lower someone's rank from one rank to another. The manager demoted Bill from cashier to clerk. The army demoted her from lieutenant to sergeant.