Before they do a national sales tax or flat tax to replace the income tax, they need to do a constitutional amendment repealing the 16th.
The fair tax act repeals all of the code sections dealing with the income, estate, corporate income and payroll taxes. You can't repeal the 16th amendment without something to replace it. The Fair Tax act specifically defunds the IRS in 2-3 years and actually orders the destruction of all tax returns in its possession. There is a companion bill which does call for a constitutional amendment to repeal the 16th, but the fair tax needs to be passed first.