So they could change a 35 year old policy of the DOJ that a sitting President can't be indicted.
They could. It would ultimately be up to the Attorney General (Sessions), so it's unlikely. The legal reasoning behind not indicting a sitting President is laid out meticulously, with the primary factor being that the Constitution already prescribes the governing body (Congress) and method (impeachment by the House and trial by the Senate) to deal with a president accused of indictable offenses. Otherwise a defiant and angry DOJ could bury the president in indictments and bring the Executive branch to a halt.
There are foir separate Justice memos on the subject. Three are “no indictment” opinions, and one “yes indictment” opinion. The three “no indictment” opinions were issued by executive-branch lawyers, while the lone “yes” opinion came out of the Starr probe, which doggedly pursued Clinton and finally got him impeached.
There is no definitive case law on it, but various Supreme Court justices over the years have made comments about it here and there.
The debates during the framing and ratification of the Constitution suggest that the president is of course subject to laws like any citizen, but never discuss prosecution while in office. During the trials of Aaron Burr, Chief Justice John Marshall had insisted that Thomas Jefferson was subject to subpoena, but also that as president he could refuse to attend court in person, and could withhold some evidence.
Justice Marshall noted, and Justice Department lawyers have agreed since then, that an indictment of a president while in office would besmirch the “symbolic head of the nation.” In addition, “only the president can receive and continuously discharge the popular mandate expressed quadrennially in the presidential election,” making an indictment or trial “politically and constitutionally a traumatic event.” Impeachment is the first line of defense against presidential misconduct, one memo author noted. “This would suggest strongly that … criminal proceedings against a President in office should not go beyond a point” that they would effectively remove a president, and thus become a short-cut for impeachment.
So the argument against indicting a sitting President is pretty sound, with "I don't like him and I want to indict him really, really bad" falling woefully short as a counter argument.
The chances of a sitting President being indicted are very, very slim. It's far easier to do it by the book and impeach him, remove him from office then throw the book at him if warranted. Otherwise, runaway indictment fever could be and would be abused to the point of shutting down the presidency, effectively removing him from office without removing him from office.