Jon Niese is a changed man. The transformation began near the end of 2011, when the Mets lefty began dabbling with a changeup. He entered last spring training sold on the pitch and dedicated to mastering it. By the end of 2012 it was nearly mission accomplished for Niese. “He’s getting very close to being perfect,” pitching coach Dan Warthen said yesterday, when asked to assess Niese’s changeup. “I think it contributed to him finishing the season strongly.” The 26-year-old Niese had his best season in the big leagues last year, going 13-9 with a 3.40 ERA with 155 strikeouts and 49 walks in 190 1/3 innings. By Warthen’s accounting, just two to three percent of Niese’s pitches were changeups in the first half of last season, but that figure increased to the 12-15 percent range in the second half. Throwing more changeups, Niese went 6-5 with a 3.01 ERA in the second half of last year, marking the first time he avoided a late-season fade. The goal now for Niese is to refine the changeup even further.