If you're an at-will employee, you may be terminated at will by your employer and the employer does NOT need to let you go back to your unionized position, regardless of whether you pay union dues or not, *unless* there is a union or collective bargaining contract covering you (and employees like you) which states that if you lose your non-union position, you have the right to flow back to your prior union position. You need to review the union/collective bargaining agreement to see what rights it gives you.