You can no more change your original disposition (which causes the million), than you can change the lesion that causes cancer.

You can control your original disposition in exactly the same way you usually control your decisions. Even normally when you consider a decision the outcome is already settled and the measure of all Everett branches involved already determined. Just because you consider the counterfactual of local miracles that result in a different decision when evaluating your preferences doesn't mean any such local miracles actually happen. Your original disposition is caused by your preferences between the two "possible" actions, just like with any other decision. The lesion example is different because your preferences are at no point involved in the causal history of the cancer.