One way around the federal estate tax is by arranging for inheritances to skip generations within a family. Instead of leaving property to their children, wealthy people would leave it to their grandchildren and even great-grandchildren. However, Congress has created a tax on these generation-skipping transfers, according to Wealth Management in "GST Tax Exemptions in Jeopardy."
Congress left open an exemption from these taxes for generation-skipping transfers from irrevocable trusts that were created before Sep. 25, 1986. However, this created another issue. If one of those old exempt trusts is modified, it is not always clear whether it continues to be the same trust or whether it should really be viewed as a new trust, which would subject the transfer to the tax. The answer depends on how the IRS decides to rule in most cases.
If you have an old, exempt trust or are the beneficiary or trustee of one, then it is extremely important that you go to a wills and trusts attorney, if you consider modifying the trust in any way.
Reference: Wealth Management (May 18, 2018) "GST Tax Exemptions in Jeopardy."