Yes, a company can decrease your pay with a move to a higher position--or a lateral transfer, or if you stay in the same position; i.e. at any time, no matter what your position--unless you have a written employment (including union) contract setting your pay. Without a contract, you are an employee at will, and one of the consequences of being an employee at will is that your pay is subject to the "will" of the employer: they can change it at any time, for any reason.