WASHINGTON — After the fiscal 2019 defense authorization act is signed into law later this month, Defense Department officials will be handed the most sweeping changes to the military officer personnel system in the last 38 years.

But how and when they’ll use those new provisions remains unclear.

Robert Hood, the assistant secretary of defense for legislative affairs, said Thursday that officials are still working through the details of the massive military policy bill, which includes promoting some officers ahead of traditional schedules and allowing others to stay in rank longer than the current up-or-out system allows.

“We’re really changing a system you grew up in, if you’re uniformed military,” Hood said.

He added that defense officials have been looking for more flexibility with the officer career planning, but he and other Pentagon leaders have thus far declined to specify when many of the new provisions will be put into place.

The changes to the 1980 Defense Officer Personnel Management Act, drawn up in the early years of the all-volunteer force, were included in the 1,800-plus page defense policy bill by senators who argued that the military’s approach to recruiting and retention needs to be modernized.

“It is important to retain the up-or-out concept for officer management, but there must be some exceptions when the needs of the military will be best served by retaining an officer,” they wrote in their explanatory language for the bill.

The changes aren’t mandatory, but have been hailed by outside advocates as necessary flexibility to attract and retain individuals with valuable technical skills.

Among the changes, services will be able to develop “alternative promotion processes” for specific career fields, with an option for some officers to stay in lower ranks without promotions for years beyond what would normally warrant a forced separation.

For rising younger talent, service officials will be allowed to spell out rules for quicker promotion schedules, disconnected from time in service.

And the changes will allow the military to bring in mid-career civilians with high-demand skills at ranks up to O-6, potentially a valuable recruiting tool for fields like cybersecurity and logistics.

Even supporters of the changes warn that the new rules could create some cultural friction within the services, but Hood said he has not gotten significant pushback on any of the provisions yet.

He said the rules “provide (leadership) some more flexibility to move some things around.” Pentagon planners met extensively with Capitol Hill staff on the issue in recent months, so none of the new authorities come as a surprise.

Most of the specific decisions will be left to the the individual services, as was spelled out in the authorization bill. And that won’t happen until after the measure is actually signed into law.

The Senate finalized Congress’ work on the legislation on Wednesday, and President Donald Trump is expected to sign it into law when it arrives at the White House later this month.