Who changed his name because he didn’t like his monogram?

The one born "Hiram Ulysses Grant."

While it is certainly true that Grant changed his name, the name by which he is now know wasn't the one he intended.

"Hurriedly Hamer made out the request, although for a moment he was stumped trying to remember the boy's precise name. The deed done, he responded to Jesse's request: "I received your letter and have asked for the appointment of your son, which will doubtless be made.

"To identify his trunk, he and his cousins hammered in his initials, but it took only a moment to see that "H. U. G." would not do: Ulysses was not going to be the butt of any more jokes if he could help it. From now on he would be Ulysses Hiram Grant.

"The adjutant firmly informed him that there was no appointment waiting for such a person. Two Grants were scheduled to arrive: Elihu Grant from New York and Ulysses S. Grant from Ohio." - Ulysses S. Grant: Triumph Over Adversity, 1822-1865
by Brooks D. Simpson

The explanation that I have seen, which fits in with the excerpt above, is that his appointment to West Point came from Congresman Thomas Hamer, with whom Jesse Grant, Ulysses' father, had a quarrel with. However, there has always been a rumor that Jesse's wife, Hannah (nee' Simpson), visited Mrs. Hamer in order to ask for the Congressman's appointment for Ulysses. Year ago, I saw a copy of the letter from Hamer to the Academy, and the name of the young man appointed was Ulysses Simpson Grant.