How did General Grant's name get changed from Hiram to Ulysses?

U.S. Congressman, Thomas L. Hamer was pretty much the cause. Hamer erroneously nominated him for the United States Military Academy as Ulysses Simpson Grant, knowing Grant's mother's maiden name and forgetting that Grant was referred to in his youth as "H. Ulysses Grant" or "Lyss". Grant wrote his name in as "Ulysses Hiram Grant" in the West Point registration ledger (concerned that he would otherwise become known by his initials, H.U.G.). West Point's registration adjutant, upon noting the discrepancy between how Grant signed himself in and what was on the official list, advised Grant that he could be registered as Ulysses Hiram Grant, but that the correction had to go to Washington City (D.C.) to be corrected so it could become official. Grant decided, on the spot, to go with the name under which he had been offically nominated by Congressman Hamer, "Ulysses Simpson Grant" but only used the "S" outside of official channels. When asked in the future what "S" stood for he replied it meant nothing in particular. And thus Hiram Ulysses Grant became Ulysses S. Grant.