" refers to a hypothetical future event. "to be to" here can be classed as a type of modal verb; it is like "if they will upset", except that in reality we don't use "will" in such cases.
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"if they are to upset ..." refers to a hypothetical future event. "to be to" here can be classed as a type of modal verb; it is like "if they will upset", except that in reality we don't use "will" in such cases.