The Senate Select Committee on Ethics yesterday changed its schedule to extend public hearings in the "Keating Five" case, possibly into January, to accommodate testimony by a key aide to savings and loan executive Charles H. Keating Jr. under a limited grant of immunity.

The move came as several committee members expressed skepticism about a banking regulator's testimony Wednesday that intervention by the five senators on Keating's behalf delayed enforcement action against Keating's Lincoln Savings and Loan Association, at "greatly increased loss" to taxpayers.

The committee on Wednesday ordered production of documents and testimony by James Grogan, Keating's lobbyist and his chief contact with the five senators who are under investigation for their ties to the failed thrift executive.

The committee appeared to underscore the importance it attaches to Grogan's documents and testimony by taking the rare step of granting limited immunity and by asserting that it could not "fulfill its obligations to these members, and to the Senate, without obtaining his {Grogan's} testimony."

Grogan is among seven company officials, including Keating, who have invoked Fifth Amendment rights against self-incrimination in refusing to answer questions from committee investigators. The grant of limited-use immunity prevents use of his testimony before the committee in any future prosecution but does not bar criminal action against anyone, including Grogan.

As a result of the surprise move to call Grogan as a witness, lawyers for the five senators -- Alan Cranston (D-Calif.), Dennis DeConcini (D-Ariz.), John Glenn (D-Ohio), John McCain (R-Ariz.) and Donald W. Riegle (D-Mich.) -- moved to delay the senators' final testimony until after Grogan appears.

The committee, which had been hoping to complete hearings by the middle of next week, announced a new schedule that could extend sessions until shortly before Christmas or, according to one panel member, until well into January.

Grogan is under orders to produce subpoenaed documents Wednesday and testify in closed session Dec. 17. He will testify in public session if his information bears on the the ethical conduct of the senators, according to committee sources. Timing of the public session depends in part on the amount of time that lawyers need to prepare, the sources said.

Under persistent questioning from committee members as well as lawyers for the five senators, William K. Black, the first regulator to assert that the senators' intervention had an effect on government action against Lincoln, conceded that he had no direct knowledge of specific moves by the senators that delayed enforcement.

But Black, who was one of four regulators summoned to meet with the senators in April 1987 to discuss the Lincoln case, continued to insist that it was his "judgment based on the facts" that their intervention stalled effective enforcement action against Lincoln. Among other things, he cited the senators' expressed interest in halting a critical examination of Lincoln's operations and finances, a probe that he said could have contributed to earlier government takeover of Lincoln and resulted in lower costs to taxpayers.