Mets pitcher Clay Holmes throws during a spring training baseball...

Mets pitcher Clay Holmes throws during a spring training baseball game against the Houston Astros in Port St. Lucie, Fla., on Saturday. Credit: Newsday/Alejandra Villa Loarca

PORT ST. LUCIE, Fla. — For Clay Holmes, this spring training is all about change — and a changeup.

He changed from Yankees to Mets. He is changing from reliever to starter. And he has added a changeup to an expanding repertoire in the expanded role.

All of the above went well Saturday, with Holmes throwing three perfect innings in a 6-2 win against the Astros, the Mets’ exhibition opener. He struck out three and threw 35 pitches (and 15 more in an extra simulated inning in the bullpen) against a lineup of mostly minor-leaguers.

Holmes called the occasion “special.”

“This was the most excited I’ve been to pitch — definitely up there,” Holmes said. “I definitely felt it coming to the park today. I tried to sit in the dugout before I ran out, just to see all the players and the energy that’s here. Just realizing I’m getting right out in the first inning. This opportunity I have with this team, it made it a special day for me.”

Holmes came away pleased that he was able to mix in all six of his pitches: sinker, sweeper, changeup, cutter, four-seam fastball, slider.

The changeup in particular, which he threw seven times against Houston, has drawn rave reviews.

“I’m not surprised by it, because that’s what we’ve seen so far in camp, whether it’s in bullpen sessions, live BPs and the feedback from hitters that were facing him here,” manager Carlos Mendoza said. “I went up to some of them like, ‘What was that changeup looking like?’ ‘That was pretty nasty.’ ”

Holmes has been in Port St. Lucie for more than a month already, part of his process of preparing for a starter’s workload. The Mets wanted to give him extra time to build strength, which they accomplished.

“We’re going treat him the same way we treat all of our starters,” Mendoza said. “He’s physically built for it. And he’ll go through the same testing and everything that we do in the training room. Yes, we’ll watch him. But so far the way he’s responding (is encouraging).”

Extra bases

Johan Santana arrived for a stay as a guest instructor. He spent time with Sean Manaea, who recently listed Santana as his favorite player as a kid . . . The Mets drew a sold-out crowd of 7,394 at Clover Park.