For two decades, Billy Beane has reigned as the A’s mastermind, the team’s one real constant.

For much of that time, David Forst has served as Beane’s trusted lieutenant, turning down chances to go elsewhere to stay in Oakland.

And for nearly seven years, Bob Melvin forged a strong relationship with the front office while taking the team to three postseasons in each of his first three years managing Oakland full time.

Next season might be the last for all three.

There is increasing speculation in baseball circles and among those who know the A’s higher-ups that — with no long-term contract extensions on the table — team owner John Fisher will decide to head in a different direction, or more likely, Beane, Oakland’s vice president of baseball operations, will take a step back to allow general manager Forst to run the whole show.

And what happens with Melvin is anyone’s guess. A’s ownership denied the Yankees’ request to interview Melvin last winter for their open managerial job, but did not offer him a new contract at the time.

Under terms of their deal, perfectly allowable. As baseball is generally conducted, though, the move was at the very least a faux pas, if not a downright insult.

“It’s crazy to leave someone with Bob Melvin’s credentials hanging like that,” one American League executive said Monday. “You’re taking a chance he might be so pissed that you wind up losing him anyway.”

Beane, Forst and Melvin do not discuss contracts, including their own, as a matter of policy, and A’s team president Dave Kaval also cited that when asked about the status of the team’s most visible leaders.

“We typically don’t address contract matters publicly, but the key things are that all three individuals have done an outstanding job, and we’re really pleased with their performance,” Kaval said. “Billy and David have put together a quality organization, and Bob has done an exemplary job getting a young team to play together — that’s something we feel strongly about.”

The A’s are undergoing a major rebuild, spearheaded by Beane and Forst. Upending the architects of the rebuild during the process would be unusual.

“Absolutely, it’s important,” Kaval said of stability at the top during a period of transition: “They’re doing a very good job, I enjoy working with them, they’ve got great energy to work through this period of time for the team — it’s the dawn of a new era here. They’re all very important stakeholders in that.”

Then again, change is the new normal inside the franchise, which has added more than 50 employees in the past year and a half, including seven new vice presidents. The latest, vice president of finance Adam Buckfelder, was hired Monday. Only Beane and vice president of stadium operations David Rinetti remain in place from the pre-Kaval board room.

The A’s are taking on more of a tech-firm vibe, with sleek new offices in Jack London Square that include a batting cage, gym and cafeteria. A few long-term employees have been let go or encouraged to go elsewhere; a few have jumped to the Giants, and many more have sent their resumes there.

Within both teams, there are whispers: With Giants executive vice president of baseball operations Brian Sabean, general manager Bobby Evans and manager Bruce Bochy’s deals also up in 2019, could one or more of the A’s bigwigs wind up with their cross-bay rivals?

Typically in baseball, a year and a half left on a deal is verging on lame-duck status. Only one year left? Basically out in the cold.

Consider Beane’s deals: In 2005, he signed a contract through 2012. On June 13, 2007, he signed an extension through 2014, and on Feb. 7, 2012, he signed an extension through 2019. On Jan. 14, 2013, Melvin was extended through 2016, and on Sept. 9. 2015, he was extended through 2018. Last year, he received just a one-year extension.

Beane has several professional commitments outside of baseball, serving on numerous corporate boards, serving as an adviser for Dutch soccer club AZ Alkmaar, and owning a share of English soccer club Barnsley. It would surprise no one who knows Beane if he wanted to pursue other opportunities, in baseball or elsewhere.

That could get tricky, however: Beane retains a small ownership stake in the A’s, and any other team he might join would by definition be a competitor.

“That’s a very complicated dynamic that doesn’t exist elsewhere,” one industry insider said. “It’s an issue if he takes a job with another team; the A’s owners would have to buy him out or allow him to sell his shares on the open market, which they’re unlikely to want to do. The commissioner potentially would have to get involved.”

Uncertainty at the highest level is almost unthinkable for the A’s after “Moneyball” anointed Beane as one of the top minds in pro sports, but it’s becoming increasingly possible to imagine a diminished role for Beane — or none at all.

“David Forst has done a very good job, and I could see Billy taking a step back,” one longtime friend of Beane’s said. “They’re both very valuable assets, but I just don’t see things going on as the way they are very much longer.”

Susan Slusser is a San Francisco Chronicle staff writer. Email: sslusser@sfchronicle.com Twitter: @susanslusser