Each district court judge shall be paid by the Commonwealth an annual salary that shall be fixed in the general appropriation acts and set at an amount equal to 90 percent of the annual salary fixed by state law for judges of the circuit courts. Each substitute judge of a district court shall receive for his services a per diem compensation of $350, except when such judge sits pursuant to the provisions of Title 37.2, in which case compensation shall be limited to that provided in § 37.2-804. The judge replaced may certify that the substitute judge is entitled to $175 if the substitute judge acted in his official capacity for less than a full court docket or served less than four hours. A full-time judge elected to an initial term after January 1, 1974, shall also be prohibited from engaging in the practice of law.