Except as specifically provided in the trust instrument or the will or in this part [Sections 62-7-401 et seq.], this part shall apply to any receipt or expense received or incurred by any trust or decedent's estate whether established before or after June 3, 1963, and whether the asset involved was acquired by the trustee before or after June 3, 1963; provided, however, it shall not be applied to upset apportionments or allocations of corporate distributions declared by the corporation or heretofore made in fact by trustees or to disturb any rights heretofore vested in the life beneficiary.