Yes, you could be, unless you have an employment contract which protects your employment in some way. Without a contract, you are an employee at will; an employee at will may be terminated--or anything short of termination, such as demoted, suspended, pay cut, transferred, hours reduced, etc.--at any time, for any reason which is not illegal discrimination. It would not seem to be illegal discrimination to terminate or demote you in order to re-employee an employee in the military, reserve, or Guard whose position you filled while they were away. If you do have a contract, however, you can enforce any terms of its which are beneficial to you.