Unfortunately, yes, employers have the right to change a severance amount or revoke your severance agreement altogether. However, this is not the case if you have a contractual agreement with your employer.
Should you have an existing contract regarding severance, or you’ve already signed the agreement in your severance package, your employer is not allowed to change terms or fail to follow through with the agreement. If they do, you may have a valid claim for breach of contract against your employer.