You can substitute 1 cup of canned sweet potatoes or yams for fresh sweet potatoes. Drain liquid and mash. Canned will not have as much flavor or natural sweetness as freshly roasted.
Can use 1/2 cup of oil or butter instead of 1/4 cup of each. I like using both oil and butter, butter for flavor and oil for moistness. I feel that if you use only butter, cornbread may not be as moist but still good!
If you want to reduce the amount of sugar in this recipe, try substituting 1/4 cup brown sugar for 1/4 cup pure maple syrup or honey. Coconut sugar is another option.