The union can promise you everything, but can guarantee nothing. All the union can do is ask. The Company has the right to say NO to any proposal or demand that the union makes.
If the union tells you that what you have now is guaranteed, it is not telling you the truth. The truth is that you can lose wages and benefits in collective bargaining. If the union were to get a contract, your wages, benefits, hours and other working conditions could be reduced, stay the same or be improved. It ultimately depends on what the Company is willing to agree to.