Matt Harvey aced his freshman finals, but now has to avoid succumbing to the sophomore slide. The Mets figure the prized right-hander has an excellent chance of succeeding, after he spent the offseason and spring training perfecting a new changeup to complement his raw heat. When the first pitch is thrown tonight at Citi Field, the Padres will be seeing a more complete pitcher than the one they hammered last August at Petco Park, in Harvey’s third major league start. Overall, Harvey finished his rookie season 3-5 with a 2.73 ERA in 10 starts for the Mets, with 70 strikeouts, 26 walks and 42 hits allowed in 59 1/3 innings. Those numbers excited the Mets, but they were not complacent in the offseason with the stud fireballer. “Matt Harvey, when the season was over, was told, ‘Your changeup is another pitch you need to use,’ ” manager Terry Collins said. “And all he did was come to spring training, worked hard and used it and became a very effective pitcher, so he has made adjustments himself, already.” Though Jon Niese carries the ceremonial title of “ace” with Johan Santana no longer in the picture, the battle for the top spot figures to heat up this summer, between the 24-year-old Harvey and top pitching prospect Zack Wheeler, who is starting the season at Triple-A Las Vegas. For now, it’s Niese, Harvey, Dillon Gee, Jeremy Hefner and Shaun Marcum filling out the rotation. Niese set the tone in Monday’s opener by holding the Padres to two runs over 6 2/3 innings in the Mets’ 11-2 victory. “All five of us are in a row, so we’re all pushing each other,” Harvey said. “We’re all, not necessarily trying to one up each other, but it’s a competitive group and we all want to do well, and obviously Niese’s strong performance [Monday] pushes all five of us to go out and pitch [well].”