S.D. Codified Laws § 55-17-4
A South Dakota special spousal trust may not be amended or revoked unless the trust agreement provides for amendment or revocation, or unless the trust agreement is amended or revoked by a later South Dakota special spousal trust. To amend or revoke the trust, the later South Dakota special spousal trust is not required to declare any property held by the trustee as special spousal property. The amended trust or the revocation is enforceable without consideration. However, notwithstanding the other provisions of this chapter, unless the South Dakota special spousal trust expressly provides otherwise, at any time after the death of the first spouse the surviving spouse may amend the South Dakota special spousal trust with regard to the surviving spouse's property to be disposed of at the surviving spouse's death. For purposes of this section, the term, surviving spouse's property, means the property that consists of the surviving spouse's property that is not South Dakota special spousal property and the surviving spouse's share of the special spousal property determined as of the date of the first spouse's death.
SDCL 55-17-4