No, they won't. The FairTax is no a VAT and cannot be made into one. It is far more simple, efficient, less costly, and transparent that a VAT.
In addition the FairTax taxes only end use consumption rather that cascading (as does a VAT). The VAT then requires the buyer to attempt to get his money back through various supplications to the government. We all have had enough of supplications to the merciful and loving IRS.
You have far more faith in elected representatives than I do.
When the dems are back in charge they'll go at the Fair Tax with hammer and tongs. Since no IRS will exist, the quickest way to raise taxes is to convert the Fair Tax into a VAT.