If there are changes in the estate laws and a person does not change their estate plan to accommodate the changes, there is a way to mitigate the risk, according to the Wills, Trusts & Estates Prof Blog in "How to Use Trust Disclaimers in Estate Planning."
The idea takes advantage of a little known part of estate law that allows an heir to disclaim any inheritance or portion thereof.
This means that an heir can say he or she does not want the inheritance and, therefore, it is given to the next person in line for it.
A trust can be written in such a way that a surviving spouse can disclaim any assets and the next "person" in line to receive them would be the trust.
This approach allows for any large assets to be disclaimed into the trust to bring the estate down below the estate tax exemption limits.
These trusts do have their own risks and drawbacks.
An estate planning attorney can advise you in creating an estate plan that meets your unique circumstances.
Reference: Wills, Trusts & Estates Prof Blog (August 24, 2017) "How to Use Trust Disclaimers in Estate Planning."