Don't look now, but Ronald Reagan has done it again. Without admitting error or acknowledging failure, the president has changed course and become an advocate of policies he previously rejected or ignored.

The instant case is Reagan's embrace of the central recommendations of the Packard Commission, which has made useful proposals designed to streamline Defense Department management and improve weapons procurement and development. Many of its recommendations dovetail with items in military reform legislation by Sens. Barry Goldwater (R-Ariz.) and Sam Nunn (D-Ga.) that will soon be taken up by Congress.

With his customary avoidance of understatement, the president claimed in a statement recently that the Packard Commission proposals "are among the most extensive reforms of the defense establishment since World War II."

What did not appear in the statement, issued by White House spokesman Larry Speakes in Santa Barbara, Calif., while the president rode the range at his nearby ranch, was an explanation of why Reagan has waited until the sixth year of his presidency to climb aboard the bandwagon of military reform.

The explanation can be found readily on Capitol Hill, where the Senate Budget Committee has voted to raise taxes and give the president about $25 billion less for defense than his budget has requested. Understandably, this action by a Republican-led committee in the Republican-controlled Senate sent shudders throughout the military establishment and the White House.

Reagan has a good case when he argues that this kind of defense cut would send the wrong signal at the wrong time to the Soviets and undo much of the defense restoration that has been a central feature of his presidency. Thanks in part to the Gramm-Rudman-Hollings antideficit legislation, the defense budget sustained a real cut last year for the first time during the Reagan administration. It is hard to see how a follow-up reduction would deter the Soviets or help the national interest.

But responsibility for the congressional revolt against the military budget rests at the White House doorstep. Against the advice of top aides and First Lady Nancy Reagan, the president has stuck with Defense Secretary Caspar W. Weinberger, who long ago wore out his welcome on Capitol Hill by claiming that even the most minor cuts in military spending represented a clear and present danger to the country.

By trying to defend everything, Weinberger wound up lacking credibility on anything. And the defense secretary's intransigence was compounded by horror stories, some real and some exaggerated, about weapons failures and extravagantly expensive hammers and toilet seats.

Reagan's response has been reminiscent of the old story about the defense attorney who maintained that his client wasn't at the scene of the crime and didn't pull the trigger if he was. While ridiculing the stories of Pentagon waste, the president also has said that his administration deserves credit for uncovering them.

Now he has taken a different tack, one characteristic of Reagan when he is in real difficulty. The last resort of scoundrels, in the famous phrase, is an appeal to patriotism. Reagan's last resort usually is to name a bipartisan commission headed by a famous figure of an earlier Republican administration.

A bipartisan commission headed by then-President Ford's national security adviser Brent Scowcroft kept the MX missile alive after it had been scuttled by the initial ineptness of Reagan and Weinberger. A bipartisan commission led by Alan Greenspan, head of the Council of Economic Advisers in the Ford administration, rescued Social Security. A bipartisan commission led by Henry A. Kissinger, while less successful than the first two, has helped give Reagan a wider range of options in Central America.

And the bipartisan commission chaired by David Packard, deputy defense secretary in the Nixon administration, has enabled Reagan to make a show of seriousness about military reform and improve the chances for a defense budget increase. It is belated, but better late than never.

Reaganism of the Week: Speaking at a fund-raiser for Republican Senate candidate Rep. W. Henson Moore (R-La.) on March 27, the president said: "There's an old saying that in raising taxes, as in shearing sheep, it is best to stop when one gets to the skin."