(taxpayer proposed to modify trust by appointing successor trustees, and IRS determined that such modification would not create a general power of appointment that would cause trust assets to be included in taxable estate of any trust beneficiary, and exercise of appointment power would not be a gift taxable transfer; IRS also determined that proposed modifications would not cause trust to lose exempt status from GSTT tax, and no distributions or terminations of trust would be subject to GSTT due to modifications).