WASHINGTON (AP) - It is time to dig into the sock drawer in search of that savings bond from Aunt Martha. The Treasury Department announced Tuesday that it has launched a program to convert existing paper bonds into electronic securities. The program, dubbed SmartExchange, isn't mandatory, but Treasury officials are hoping that holders of the existing 760 million paper bonds will take them up on their conversion offer. "Exchanging paper for electronic securities marks another step in moving savings bonds from a paper-based, labor intensive program to an all-electronic product," Van Zeck, the Treasury's commissioner of the public debt, said...