The Reagan administration has altered its tax proposals significantly, lessening the scope of the corporate minimum tax, increasing the levies to be applied to the insurance industry and reducing the number of multi-year contracts to be covered by tougher tax regulations.

Until yesterday, the administration had never fully disclosed details of tax increases, most of which are to go into effect at the start of next year but, over the past month, the legislative initiatives have been altered substantially during private negotiations within the Treasury Department. Among the most important shifts are:

* The corporate minimum tax. The administration decided to weaken the severity of the proposal by giving corporations paying the alternative tax a credit that can be used to reduce taxes owed in future years.

Under the proposal, a corporation paying little or no tax because it used certain tax preferences would have to pay an alternative minimum tax. The alternative tax would replace the existing "add-on" minimum tax and would broadly expand the number of companies subject to the tax from 5,500 to 90,000.

The credit would be the difference between the tax owed under regular calculations and the amount required under the new system. In terms of revenues, however, the shift is largely responsible for a reduction in projected taxes from $23.8 billion through 1987 down to $19.1 billion.

* Multi-year contracts. The administration is calling for the elimination of regulations and legislation allowing major contractors, defense suppliers and the aerospace industry to defer payment of taxes until the completion of the contract. In the case of a major weapons system, this tax deferral can mean a lucrative postponement of tax liability for as long as 15 or 20 years.

The administration, however, decided to exempt existing contracts, a step that is largely responsible for a reduction in anticipated revenues from $3.3 billion in 1983 to $1.9 billion, and from $5 billion in $1984 to $4.4 billion. In later years, revenues will be larger as deferred taxes from existing contracts become due.

* Insurance companies. Under existing law, insurance companies can enter into joint coverage arrangements which, under the tax code, function to convert investment income, which is taxed at a 46 percent rate, into underwriting income, which is taxed at 23 percent.

The administration intends to end this tax break, and over the past month decided to accelerate the effective date to cover the insurance industry this year. The effect is to raise $900 million in 1982 and $1.9 billion in 1983 compared to original plans limiting revenues for those years to $1.1 billion in 1983.

At a briefing yesterday, John B. Chapoton, assistant secretary for tax policy, defended the tax proposals against charges that the new legislation will take back much of the corporate tax breaks in the Reagan Economic Recovery Tax Act of 1981.

Chapoton contended "the proposals do not undermine in any way last year's proposals.

Emil Sunley of Deloitte Haskins and Sells, among others, has cited the fact that in terms of corporate income tax liabilities the current Reagan tax proposals would increase corporate taxes by $9.1 billion in fiscal year 1983 and $16.1 billion in 1984. The 1981 proposal cut corporate taxes by $13.1 billion and $21.6 billion in those years.