Subject to the rights of creditors and taxing authorities, competent successors may agree among themselves to alter the interests, shares, or amounts to which they are entitled under the will of the decedent, or under the laws of intestacy, in any way that they provide in a written contract executed by all who are affected by its provisions. The personal representative shall abide by the terms of the agreement subject to his obligation to administer the estate for the benefit of creditors, to pay all taxes and costs of administration, and to carry out the responsibilities of his office for the benefit of any successors of the decedent who are not parties. Personal representatives of decedent’s [decedents’] estates are not required to see to the performance of trusts if the trustee thereof is another person who is willing to accept the trust. Accordingly, trustees of a testamentary trust are successors for the purposes of this section. Nothing herein relieves trustees of any duties owed to beneficiaries of trusts.