The change does basically seem feasible. Reducing box 1b would increase your tax. But I don't know how TurboTax would have calculated the tax before the correction, because box 1b should not have been more than box 1a. If you had left the incorrect entry you would have gotten an error message in the federal review.
The amount of box 1a minus box 1b is taxed at the rates for ordinary income. The amounts in boxes 1b and 2a are taxed at the lower rates for qualified dividends and long-term capital gains. Neither of them is a fixed percentage. The percentage depends on your other income and your filing status. And if either type of income happens to cross a bracket boundary it will not all be taxed at the same percentage.
