AS PRESIDENT REAGAN'S tax reform bill moves slowly through Congress, the modifications so far have improved it. The House Ways and Means Committee finished work last weekend and, while it made a number of significant changes, none of them really challenges the basic outline of the bill as Mr. Reagan proposed it last May. Yet now the White House treats it as an open question whether Mr. Reagan will grant the Ways and Means version even the minimal support necessary to put it before the Senate. While the chairman of Ways and Means, Rep. Dan Rostenkowski, has by heroic exertions managed to move the bill one long step closer to reality, the administration's enthusiasm seems simultaneously to be cooling.

Like the president's original proposal, the Ways and Means revision lifts the tax threshold above the poverty level of income. Some 6 million people who earn very little would no longer have to pay income taxes on it, an adjustment that is doubly urgent because of the rising burden of payroll taxes on these people. It is worth noting that this change is simultaneously the most important reform in the bill and the least controversial.

In another contribution to fairness, the Ways and Means Committee has also been less generous to the people at the top of the income scale than Mr. Reagan's bill was. But this is no soak-the-rich bill; the wealthy would still harvest a greater increase in after-tax income than people farther down the ladder -- although not so much greater as the president originally intended.

In one of its few conspicuous departures from Mr. Reagan's bill, Ways and Means would retain full deductibility of state and local taxes. That doesn't have much to do with fairness, but it will greatly ease the financing of public education, a national responsibility that takes precedence over most of the things that the federal budget supports.

The central political issue in this bill, and its chief purpose, is to reduce individuals' taxes by reducing the rates and to swing that burden onto corporations by abolishing some of their present breaks. Under present law, over the next five years individuals would pay 83 percent of total income taxes and corporations would pay 17 percent. President Reagan's original bill would have cut individuals' share to 78 percent and raised corporations' to 22 percent. Ways and Means follows the same pattern, going half a percentage point farther in the same direction. As they consider the implications, corporations' opposition to the bill is steadily increasing. The White House blames it on the Ways and Means changes.

But the Ways and Means bill does not greatly depart from the model that Mr. Reagan proposed six months ago. It is more progressive in its impact on rich and poor, but not fundamentally different in its balance between private individuals and business. The question at the White House is, apparently, whether to proceed with any tax reform at all.