Friday was a day of major upheaval for the Islanders, who lost three longtime members of their core and brought in two seasoned veterans in a big shakeup of their forwards on day one of NHL free agency.

Headed out were Kyle Okposo, who signed a seven-year, $42-million deal with the Sabres; Matt Martin, who joined the Maple Leafs for four years and $10 million, and Frans Nielsen, the only one of the three the Islanders were hoping to bring back, who jumped at a six-year deal for $31.5 million with the Red Wings.

Heading in are two-time Stanley Cup winner and recent Jets captain Andrew Ladd, 30, who signed a seven-year, $38.5-million deal — he’s likely to man the left side with John Tavares — and Jason Chimera, 37, who signed a two-year, $4.5-million deal after spending the last five seasons with the rival Capitals.

“Obviously, we thank all three of those guys,” general manager Garth Snow said of Okposo, Martin and Nielsen, who have a combined 1,573 games in the Isles’ organization. “They were drafted here, developed in our system, they were big parts of some recent success. I wish them the best of luck. We’re extremely happy with where we’re at today. We feel we’ve gotten better. It’s not a knock on anyone that was here last year. We really have a positive feel on where this team’s going.”

Ladd got the full recruiting tour on Thursday and made a quick decision when the market opened at noon Friday. After winning Cups with the Hurricanes in 2006 and the Blackhawks in 2010 and spurning a long-term extension from Winnipeg before last season, he sees the chance to play with Tavares and the Islanders as a real opportunity.

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“There’s a lot of things that go into winning a Stanley Cup, and they’re off to a great start,” Ladd said. “I’m beyond excited to have a chance to play with someone of John’s caliber. I played with [Jonathan] Toews and [Patrick] Kane, those generational players, and he’s one of those guys. I can’t wait.”

Soon after that, the Islanders added Chimera, who tied a career high with 20 goals last season for the Capitals, the top team in the regular season. Even with 951 NHL games under his belt, Chimera believes he can bring speed and size to an Islanders lineup that lost both of those things with Nielsen and Martin gone.

“I’m pretty motivated to win, that’s all I really cared about,” Chimera said. “I wanted a team that had a chance to win, and the Islanders have the best chance of that. It was a no-brainer for sure.”

Okposo and Martin didn’t have to make tough decisions on whether to stay or go, given that it was clear to both longtime Islanders at the end of the season that there would be no agreement with the team.

But Nielsen and the Isles had contract talks up until late Thursday night, when Nielsen’s camp informed Snow that the longest-serving Islander would be moving on.

“After the season, I truly didn’t believe I would leave. I thought I’m going to retire an Islander,” Nielsen told Newsday on Friday. “But the summer went on and this was my only time to ever be a free agent, I wanted to see what was out there and try it even though I still didn’t think I was going to leave.

“Detroit, when they called, there’s just something special about that place. I love everything about New York — Garth, [Jack Capuano], they gave me the opportunity to play in this league, I owe them so much. But when you’re in one place so long, sometimes you can get too comfortable. I just needed a new challenge.”