Estate tax experts warn that married couples and grandparents who do not update their wills or living trusts may not benefit from the recent adjustments to the tax law. According to estate planners, President Bush's $1.35 trillion tax cut could take a big chunk out of the inheritance of some heirs. Changes to the tax laws specifically affect credit shelter trusts, which married couples traditionally use, and generation-skipping trusts, which grandparents use to pass money to their grandchildren. The scenario is even worse if individual retirement accounts are involved because heirs could be left with a hefty tax bill.