It was billed as "a tax reform law," and one of its promise was to cut the inheritance taxes paid by most Americans.

Yet since the bill was signed eight months ago, the major beneficiaries have been lawyers who, according to a "conservative estimate" given at an American Bar Association seminar, will earn as much as $4 billion from Americans who have to change their wills.

"I think my estimate is on the low side," George Haupfuhrer of Philadelphia, chairman of the ABA's section on real property, probate and trust law, said yesterday.

The new law is so complicated, he said, that old people didn't want to bother changing wills. 'They said, 'On, the hell with it. If the government gets it, it gets it.' But that's changing now. We are getting the second wave," Haupfuhrer said.

Robert C. Brown, executive director of the Tax Foundation, had to change his will as a result of the new law. He called its estate provisions, "another attorney and accountant relief act."

"Some people will spent more money to get their wills redrawn than their estates will pay in death taxes," he said.

"All our attention is directed toward the tax impact of the new law, but not the monetary costs to comply with it. They are costly items."

This led Luther J. Avery, a San Francisco lawyer who specializes in estate planning, to suggest that economic impact statements be required before new tax laws are passed to detail the cost of administering and complying with them.

Getting wills drafted is expensive. Even a simple one - involving little or no estate planning - costs $100 to $200 "if it's done carefully, and you will be lucky to get it at that price," said Paul A. Wolkin, executive director of the American Law Institute.

Getting a will drafted correctly to take advantage of the new law can mean a great deal of difference in inheritance taxes.

William C. Clay Jr., a Kentucky expert in estate law, cites an example where the federal death taxes on a $400,000 estate could be as high as $15,800 if a husband and wife are drawn one way or way as low as $800 if they are drawn another way.

Wolkin said a complicated will, involving tax-saving trusts, can run 60 pages and costs thousands of dollars. One San Francisco law firm told the Tax Foundation's Brown that its lawyers charged $2,500 for most wills they draw.

"It's a time consuming operation, especially since the lawyers are dealing with a strange law," said Wolkin. "They are familiar with the old law and had reprinted forms to work with, but now many lawyers will have to do a real education job to get to their former proficiency."

This need for re-education spawned a new industry of legal seminars on the new tax laws. The American Bar Association, for instance, sponsored three of them, including one last October that drew a standing-room audience at a New York hotel ballroom.

Even with the seminars, not all lawyers agree on what the laws provision means. Earl Colson, a Washington expert, for example, differs from most other estate specialists in feeling that the majority of wills need not be changed.

"It's unnecessary for people to panic," he said. "Only a minimum number of people are affected. The vast majority of wills, if well drawn, will serve their purpose."

On the other hand, Clay said "every will should he reviewed as a consequence of the tax reform act."

Even if a will is not changed, it costs money to have alawyer go over the will.

"There probably was a lot of legal fees by saying, 'do not change the will,'"said a congressional committee staff member who helped draft the new tax law.

As a result of these differences among experts, San Francisco's Avery expects thousands of suits over estate taxes to hit the court in 1981 - when will of people who died this year will finish probate proceedings.

He advised lawyers to increased the malpractice insurance before handling estates.

"The tax reform act of 1976 has multiplied the risks of lawyers tremendously," he said, by giving the attorney optional methods to choose from.

Most lawyers agreed with Clay that all wills need a review, but many debate the ethics of notifying clients whose wills they have drawn up - especially if there had been no contact in years.

"All lawyers are aware on the one hand that they have an obligation to a client and on the other hand of the bans against advertising," said one Washington's specialist.

Most lawyers came down on the side of telling clients their wills should be checked - usually by means of a form letter.

The provisions of the new law benefitting most Americans increased the size of estates that could escape taxes from $120,000 last year to $425,000 when the provisions are fully effective in 1982.

This involved complicated changes in the amount of an estate that can be passed to a spouse without being liable to taxes. Under the old law, $60,000 in the estate was tax exempt and another $60,000 could be left tax free to a spouse. By 1982, the new law will allow $175,000 tax free plus a marital exemption of $250,000 or half of the estate, whichever is greater.

Because of the element of choice, the way a will is drawn determines the amount of inheritance tax. Merely giving a spouse half an estate could cut the tax advantage.

There is no question that the new law closed loopholes the very rich have used to allow them to avoid inheritance taxes - as it was intended to.

For one thing the new provisions outlaw generation skipping trusts - a way to protect a child from having to pay inheritance tax while still getting its benefits.

Trust in effect at the end of last year remain intact until 1982 as long as a will is not changed substantially - leaving lawyers with a problems of deciding whether the reason for changing a will is worth knocking out the trust.