If the change referred to in subsection (a) involves a change in the highest rate of tax imposed by section
1 or
11
(b), any reference in this chapter to such highest rate (other than in a provision imposing a tax by reference to such rate) shall be treated as a reference to the weighted average of the highest rates before and after the change determined on the basis of the respective portions of the taxable year before the date of the change and on or after the date of the change.