The main provisions of President Reagan's tax plan have been known for some time. But, like any comprehensive tax measure, the proposal is filled with fine print that would change family budgets, redraw corporate balance sheets and take money from the pockets of some while returning it to others.

It would affect almost every segment of society, from preachers to companies operating in Puerto Rico to defense contractors to candidates for public office, according to documents released yesterday.

The plan would effectively redistribute a large segment of the tax burden from individuals to corporations, although Reagan has said repeatedly that the corporate income tax is merely passed along to the public.

The shifted burden -- individuals would pay 5 percent less and corporations 23.3 percent more taxes in 1986 -- is so sharp that the administration apparently decided to soften the figures by using the projected shift for 40 years from now, the year 2025.

Treasury Secretary James A. Baker III and other officials acknowledged that the figures released by the administration showing a 7 percent tax cut for individuals and a 9 percent tax increase for corporations were more theoretical than practical.

This effort, as well as other administration efforts to place the plan in the best light, comes from the knowledge that other tax-revision plans over the years have been picked apart by well-organized segments of society that felt they were being treated unfairly.

"What we hope to prevent from happening is a piecemeal picking away and destruction of the proposal," Baker said.

In general, the Reagan plan would not change the distribution of the tax burden among various individual income groups by much. The rich, poor and middle class would shoulder roughly the same share of the tax burden they do now because of a tradeoff between lower rates and the loss of certain tax advantages. But several of the proposals would have a marked impact on people within those groups.

The tax-paying poor, for example, would benefit from higher personal exemptions, higher standard deductions and an increase in the earned-income tax credit, which benefits poorer workers. Working in the other direction, however, is a change in the child-care tax credit -- which would become a deduction, usually less valuable -- and the proposal to tax unemployment benefits fully.

The middle class could be affected by an end to the deduction for two-income families, intended to ease the "marriage penalty" that puts such families into high tax brackets. Middle-income earners also could be affected by proposed taxes on part of employer-paid health insurance premiums and by the ending of their right to deduct charitable contributions if they don't itemize deductions. Other fringe benefits would be taxed, including employer-provided commuting services and employe awards.

Wealthier taxpayers could find it more difficult to shelter income by setting up trusts for their children. Under the new plan, those trusts would be taxed at the parents' rate, although money from other relatives, other sources and from the death of a parent would be taxed at the child's rate if it is kept in a separate account.

Upper income earners also would be hit by limitations on tax-deferred savings plans, called 401 (k) plans. Those would be subject to an $8,000 yearly limit, and contributions to tax-deferred Individual Retirement Accounts would have to be counted against that limit as well.

Lurking in the current tax code are thousands of provisions that may be changed, and critics of Reagan's plan are expected to wage spirited debate. Large states and cities have already begun lobbying against proposed elimination of deductions for state and local tax payments.

Companies operating in Puerto Rico and other U.S. possessions now have a tax credit that effectively wipes out taxes. The new plan would replace that with a tax credit for part of the wages paid to workers, although the transition period is longer and the credit's design more generous than proposed in the original Treasury Department plan.

Defense contractors would have to deduct certain costs over the life of a contract rather than writing them off quickly while paying taxes slowly. And those running for political office would no longer benefit from the tax credit for political contributions, which covers up to $50 a year per contributor.

Ministers whose homes are provided free would be more fortunate. The original plan to have them pay tax on an assumed market-rent amount value for those homes was removed in the Reagan version.

Baker apparently plans to forget which of these proposals were made under his predecessor, Donald T. Regan, and which are part of the president's version. Baker told reporters yesterday that the plan should be evaluated on its merits and not in comparison with "some other proposal."