Rule 465.2.c.4.a
If the damage assigned to a Unit is modified to or replaced by an amount of damage larger than the initial value, the assigning player must choose the minimum applied value such that the unit would take lethal damage. Example: A unit with 3 [M] is being assigned damage in the combat damage step. There are other units without damage assigned to them with the same controller. The unit has a delayed replacement effect applied to it that reads “Double all damage that would be dealt to it this turn.” When assigning damage, the assigning player can only choose to assign 1 or 2 damage to this unit—when doing so, the assigned damage is doubled to 2 or 4 damage respectively. The minimum applied value such that the unit would take lethal damage in this way is 4 damage.