There's no die roll involved - when a player spends a Monster Die out of their Fate Pool (page 71), they can change the target of a monster's Shoot or Move to Attack to _any_ different legal target (so the only restriction is that the new target would have to be in range and LOS of the monster).
You can also use the Forced Attack rule (on page 54) to intercept any Move to Attack (including that of a monster) to substitute a different "supporting" figure in the melee rather than the original target. The melee still happens, but with an alternate opponent for the charger. This rule is useful, for example, if a healthy figure is protecting an injured comrade.