AMERICAN PRESIDENTS detest changing their minds in public. But, awkward though the exercise might be, it's preferable to sticking with a bad position. The financial markets are now assisting Mr. Reagan in his decision to review defense spending. They find Mr. Reagan's forecasts of the budget deficits to be unrealistic as long as the administration holds to its past projections of defense spending and Social Security.

The administration apparently intends to launch a Social Security bill soon after Congress reconvenes next week. Now the White House says that defense outlays are going to have to be scaled down as well. The figures are not precise, but they establish a general order of magnitude.

The changes announced this week won't make much difference in spending during 1982. The defense budget runs with tremendous momentum, and substantial changes can be imposed only over a period of several years. The Reagan defense budget for 1982 is larger by only a small margin--about 2 percent--than the figure that President Carter proposed last winter just before leaving office. For the immediate future, Mr. Reagan's willingness to cut back on defense is important chiefly as a conciliatory gesture toward those senators, Republican as well as Democratic, who are uneasy about last spring's extraordinarily high estimates. The cooperation of those senators is going to be essential to the administration this fall in getting other kinds of spending cuts--in Social Security, among other things--that have earlier effects.

For the fiscal year 1984, the White House is evidently now proposing a level of defense spending that is almost exactly the same as that in the Carter budget last January. The original Reagan program in March called for an annual increase in defense spending, throughout his term, of about 8 percent a year in real terms--that is, after inflation. He is now evidently proposing to bring that rate of increase down to about 6 percent, the track that the Carter administration was on.

No doubt some of the Democrats will charge him with copyright infringement, while some of the military--and, especially, military contractors--will cry betrayal. But there's a sense of reality to the revised Reagan numbers that the earlier ones lacked. In view of the Carter budget, they can also claim to be based on a broad bipartisan consensus--not necessarily a bad thing in national defense.

The White House is inclined to blame this revision on poor performance by the American economy. That is one accusation the economy does not have to bear. The original Reagan budget was based on economic forecasts that were far too optimistic, and everybody knew it. If anything, the behavior of the economy is currently more satisfactory than most people expected six months ago. Who, for example, thought then that unemployment would fall this summer? The arithmetic showed from the beginning that there was not room for both Mr. Reagan's tax cut and Mr. Reagan's defense increase. The tax cut has been enacted, and defense spending inevitably must be fitted to it.

American defense policy has repeatedly got into trouble by putting too much emphasis on budget totals, and not nearly enough on how that money will be spent. The Reagan defense figures of last March were worked up in great haste, with no clear sense of program requirements, to indicate a certain direction and intention. The new numbers reflect not only economic necessity but also a clearer judgment of what's actually needed and how fast it can actually be achieved. Military strength depends on not only the number of dollars available but also the wisdom with which they are spent. The dollars will be adequate. Wisdom is always the scarcer commodity.