Mo. Rev. Stat. § 473.150: Remaining executor or administrator to continue.

If there is more than one executor of an estate, and the letters of part of them are revoked or surrendered, or part of them dies, those who remain shall discharge all the duties required by law respecting the estate, except that the court, if it finds same to be in the interests of the estate, may revoke the letters testamentary of the surviving executor, and issue letters of administration with the will annexed to the survivor and some other qualified person to whom administration could have been granted if original letters had not been issued.