The new tax law makes all your estate-planning obsolete. Almost everything has changed, and for the better. Most Americans will now be able to get away estate-tax free.

But there's a warning: If you made a will and do not change it to conform with the new law, your estate may lose out on the new tax breaks.

Say, for example, that your will says, "I leave my spouse the maximum marital deduction," which is now $250,000 or half your estate, whichever is larger.

The new law says you can leave all your property to your spouse estate-tax free. If you don't change your will to make it clear that the new rules govern, the marital deduction may be figured under the old law, according to Peter Elinsky of Peat, Marwick, Mitchell and Co. (Of course if you want the old law to govern, leave that provision of your will alone.)

Another warning: If your net worth is less than $175,000, you did not owe estate taxes under the old law and will not under the new law, either. But you still need an up-to-date will.

If you have no will, or your will is an old one, put down this column and call a lawyer. Only with a properly executed will can you make absolutely sure that your property passes to the right people. If you do nothing, your property will be distributed according to state law, which may bypass some people you meant to provide for.

And one last warning: Try not to die in 1981. The favorable new estate-planning laws start to take effect in 1982. Lawyers will want to read and reread the statute before coming to too many conclusions. But a few things seem clear:

* The amount that can be left to heirs, untaxed, will gradually rise over the next six years. This year, a single person can leave $175,000 tax-free. By 1987, he or she will be able to leave $600,000.

* A spouse can leave the other spouse any amount of money or property tax-free. For example, you can leave $1 million in stocks and bonds, or a family farm or business worth $5 million. No estate tax is due until the second spouse dies (after which everything over $600,000 is taxed). This opens the door to leaving $1.2 million tax-free. A simple example:

A wealthy person puts $600,000 into trust for the children (untaxed, because of the estate-tax credit) and leaves the rest to the spouse (also untaxed, under the new law). The spouse may receive interest and principal from the trust, if it's needed. When the spouse dies, the children can be left another $600,000 tax free -- $1.2 million in all.

The new law also gives the first spouse more control of money left to the surviving spouse. The first spouse can put the money in trust, and direct that when the second spouse dies, the remainder go to their children -- thus cutting out a possible second marriage. (The same thing could be done under the old law, but its cost was the marital deduction. The new law generally allows the marital deduction, even though the first spouse takes ultimate control of the property away from the surviving spouse.)

* Starting next year, you may give up to $10,000 a year to each of as many people as you like without paying gift taxes on the transfer (and up to $20,000 if the gift is made jointly with your spouse). This year's limits are $3,000 and $6,000.

With large gifts, a wealthy couple may transfer large sums to the younger generation, tax-free, and perhaps eliminate any tax on the parents' estates. (You might want to put large gifts in trust, Elinsky suggests, so as to control the age when the money is actually distributed to the children.)

There is no limit at all on the amount of money you can give every year to pay for someone's educational or medical expenses. The only proviso is that the money be paid directly to the college, doctor or hospital, rather than to the beneficiary.

* Whatever property is left to tax, for America's wealthiest few, will be taxed in lower brackets than formerly was the case.

Many people carry big life insurance policies to cover estate taxes. This insurance may no longer be necessary. Wealthy couples, however, might consider a joint life-insurance policy. It is written on both lives, but pays off only when the second person dies.