The fact is that in an "at will" employment relationship, absent legally actionable discrimination,  a company can set the conditions of the workplace much as it sees fit. This includes who to demote, when and why. This is true unless a worker has protection against such a change under the terms of a union agreemnt or employment contract. Bottom line, your employer is under no duty to inform you of a demotion. That having been said, if the demotion involves a pay decrease, then you must be informed of this and you must be informed before the reduction takes effect. You cannot have your wages lowered after the fact; a retroactive pay decrease is illegal. Such a decrease can only be for work to be performed going forward.