With the extortion trial of two aides to Boston Mayor Marty Walsh scheduled to begin early next year, federal prosecutors have quietly changed elements of the case against the duo, possibly to guard against reversals suffered in U.S. courts in other political corruption cases.

A "second superceding indictment," filed late Wednesday, includes at least two changes to the original charges filed against former Boston tourism director Kenneth Brissette and Timothy Sullivan, a former senior adviser to Walsh.

Brissette and Sullivan are accused of using their positions to illegally pressure organizers of the Boston Calling music festival into hiring union labor by, among other things, obstructing the issuance of permits to the festival. 

The reasoning behind federal prosecutors' superceding indictment is not clear: The US Attorney's Office made no public announcement of the new indictment, and spokesperson Christina Sterling said only that the office would not comment on matters related to a pending trial. 

The U.S. Attorney's Office confirmed late Wednesday that the new indictment had been returned by a grand jury. 

But even seemingly-small changes to the charges originally brought against Brissette and Sullivan could open a door for the defendants to challenge the entire case against them, said Harvey Silverglate, a criminal defense lawyer not associated with this case. 

And, he said, the change could signal a defect in the original case, especially if the government is responding to more recent court opinions.

"If the government has seen fit to make this change in the indictment it might very well be in response to some court opinion that [suggested that] the original indictment was defective," Silverglate told WGBH News.

Attorneys for the defendants did not return calls or declined to comment.

The new indictment neither brings new criminal charges nor changes the basic nature of the charges already brought: Both defendants remain charged with two counts of criminal extortion under the Hobbes Act. In fact, the documents appear largely identical. 

But there are at least two significant differences.

To prove extortion, the government must prove that the object of the alleged extortion was "property" -- a broad term which can include wages -- wages, for example, for union members.

The original indictment against Brissette and Sullivan cites that the "property" involved was "wages for imposed, unwanted and unnecessary and superfluous services."

The new indictment leaves out many of those words, citing instead: "wages and employee benefits."

Just two months ago, the United States Court of Appeals for the First Circuit, in Boston, considered the same words very carefully in its opinion throwing out parts of the conviction of union members accused of extortion. 

The court ruled that a lower court erred in allowing a jury to convict the defendants using a standards that included "superfluous or imposed, unwanted" work. "We do not see how peaceful picketing in pursuit of turning around jobs to maintain the prevailing wage can be deemed activity in pursuit of an illegitimate labor objective," the court wrote in its opinion.

Last month, federal prosecutors failed to obtain a guilty verdict against Democratic Senator Robert Menendez of New Jersey when the jurors were unable to reach a verdict, causing the judge to declare a mistrial.

Last year, the U.S. Justice Department abandoned its case against  Robert McDonnell, a former governor of Virginia, whose career was upended by a corruption  conviction that the  United States Supreme Court  overturned.

And closer to home, and also last year,  The First Circuit overturned three convictions at the heart of a 2010 Massachusetts corruption scandal.