Sorry, I overlooked your careful usage of "preferred" seating.

Different federal agencies have different rules on required booking with a specific travel agent, such as CWT SATO.

When a reservation is under control of a travel agent, UA will not allow substantive changes to the reservation by its own agents.

I am not sure if seat assignments, which are not supposed to have a charge, fall into that category.

I would call up UA, and if that doesn't work, call up CWT SATO.