"A/B" trusts can provide many different distribution provisions. Perhaps most typically, the maximum amount of the deceased person's interest in the married couple's property that can pass free of estate tax will be allocated to a new, separate "sub-trust" called a "bypass," "exemption," or "credit" trust. That trust is irrevocable - cannot be changed by the survivor (here your grandmother). All assets in that irrevocable trust will escape estate taxation upon the survivor's death. The survivor may have rights in that irrevocable trust to income and perhaps powers to invade it's principal for certain needs (e.g., health, education, support, and maintenance). Most typically, everything else owned by the married couple will remain in a "surviving spouse's trust" which continues to be revocable by the surviving spouse. The trustee will do an "allocation" of assets between the revocable portion and the irrevocable portion of the trust.
I have said "most typically" because your grandparents' trust is not necessarily like that - the trust needs to be reviewed carefully. Sometimes there is a third trust (a "qualified terminable interest property" or QTIP trust or marital trust), or there could be a "disclaimer trust."
If your grandmother has no desire to change any of the provisions of the trust, she doesn't need to do a new trust. She can (but is not required to) record a notice of your grandfather's death and the fact that she is now the sole trustee. She does need to give a notice to the Assessor of the county in which the real property is located that your grandfather has passed. Those forms can be obtained at the County Assessor's website.
To make maximum use of her trust, she should transfer title to her other assets into that trust. By doing so, probate may be avoided when she dies.
One last point. I'm not certain if your grandparents' estate is large enough for this to be an issue, or if your grandfather died in 2010, but if both of these things have positive answers (sufficient wealth and died in 2010) there is another issue - one that must be resolved by a week from today.
Answered on Jan 10th, 2012 at 1:27 PM