In determining the value of a person’s estate immediately before his death changes in the value of his estate which have occurred by reason of the death and fall within subsection (2) below shall be taken into account as if they had occurred before the death.

A change falls within this subsection if it is an addition to the property comprised in the estate or an increase or decrease of the value of any property so comprised, other than a decrease resulting from such an alteration as is mentioned in section 98(1) above; but the termination on the death of any interest or the passing of any interest by survivorship does not fall within this subsection.