UPPER EAST SIDE?Yesterday the Landmarks Preservation Commission approved some big changes to an UES townhouse, but the new owners of 120 East 71st Street?who bought the five-story house for $5.7 million last summer?weren't so lucky. The proposed changes included new windows and a brand new front stoop. The LPC sent the design team back to the drawing board, especially on the big stoop issue. The 120 East 71st peeps tried to cite the gigantic stoop on the building next door as precedent, but here's the thing: A bunch of folks apparently think that one's a jacked-up monstrosity. Wanna see it?