Plans change. In baseball and life.

The Mets’ Plan A was for Yoenis Cespedes to carry the offense. That’s why he was given a four-year, $110 million deal by Sandy Alderson.

Plan B involved Jay Bruce and his three-year, $39 million contract. Plan C included Todd Frazier and his two-year, $17 million deal to be a big part of the offense.

The Mets now are working on Plan D and beyond. Cespedes (hip) hasn’t been seen since mid-May. Bruce (hip) was put on the disabled list in mid-June and Frazier is plugging along after a hamstring injury. Juan Lagares (toe) was useful versus lefties.

That means four young players who were supposed to contribute to the Big Picture are now in the center of that Big Picture — left fielder Michael Conforto, center fielder Brandon Nimmo, shortstop Amed Rosario and first baseman Dominic Smith.

“All their at-bats are more competitive,” hitting coach Pat Roessler told The Post.

Here’s the most difficult part that people need to wrap their heads around. These players are so young that with the adjustments made, “you have to be careful that you don’t push too fast too soon with these guys because now you are overwhelming them,’’ Roessler said.

This has put incredible pressure on Roessler and assistant hitting coach Tom Slater, but both men have rolled up their sleeves. Progress is being made. Mickey Callaway & Co. have focused on bringing these four young players along, knowing it will pay dividends down the road.

These Mets have no other choice but to stick to this plan and work it hard. Here is what they are trying to do for these hitters.

For Rosario, one of the big goals is not to chase sliders out of the zone and his two walks in the Mets 5-2 win over the Marlins Sunday was a positive step. His chase rate has dropped considerably, Roessler said.

“He’s not going to be Joey Votto next week,’’ added the no-nonsense Roessler. “It takes time.”

The Mets head to Toronto for a two-game series where Smith, who has been slowed by a wrist injury, will DH.

Nimmo, after a red-hot start, has cooled off considerably, and the goal now is for him not to guess at the plate, a trap he has fallen into recently.

Work everything off the fastball and react to the breaking stuff. Also he has made huge progress against lefties this season and that bodes well for the future. He is hitting .271 versus left-handed starters with a .377 on-base percentage.

“When I’m going well I will sit on that fastball,” Nimmo said, “and then when I get something hanging I will fire on that.’’

As for Conforto, he needs to use the entire field. Be on time for the fastball and that will make a world of difference.

Yes, progress is being made as he has become more selective and owns a 15 percent walk rate for the year, ranking ninth in the majors. Rosario has walked six times in the last three games. The first goal for young hitters is don’t get yourself out.

Over his last 15 games Rosario owns a .293 batting average and .370 on base percentage. He also has had a break during that time when he sat down and worked on his hitting while missing some games, something Callaway also has done with Smith — spring training in the middle of the season because so much work has to be done.

All that is part of the new plan, make progress with the young hitters and if they have to miss games for that progress to be made, so be it.

One of the changes for Smith has been moving him up on the plate and he thinks that will help. He had a pinch-hit appearance Sunday, flew to right but put a charge into the ball and pulled it.

“That ball was hit at 97 [mph], I hadn’t hit anything above 94 recently so that’s a good sign,’’ Smith said.

“Dom is taking a better swing path to the ball,’’ Roessler said. “He’s not tilting over as much either.’’

For all these players it’s about the quality of the at-bat now, not the results.