Section 73 (1)
The Independent National Electoral Commission shall review the division of States and of the Federation into Senatorial districts and Federal constituencies at intervals of not less than ten years, and may alter the districts or constituencies in accordance with the provisions of this section to such extent as it may consider desirable in the light of the review.
(This simply means that INEC is authorized by virtue of this Constitution to review the division of states and federation (Federal constituencies and Senatorial districts) at intervals of not less than 10 years. And after each review, they may alter i.e. change such Federal constituencies or Senatorial district in a manner that it considers desirable according to the provision contained in this section.)
Section 73 (2)
Notwithstanding subsection (1) of this section, the Independent National Electoral Commission may at any time carry out such a review and alter the districts or constituencies in accordance with the provisions of this section to such extent as it considers necessary, in consequence of any amendment to section 8 of this Constitution or any provision replacing that section, or by reason of the holding of a census of the population, or pursuant to an Act of the National Assembly.
(In addition to what was stated in sub section 1 of this section, the Independent National Electoral Commission may at any time alter the Federal constituencies and Senatorial districts to an extent it considers necessary. This however can happen where the following situations occurs; an amendment of section 8 of this constitution – which talks about creation of states or any provision replacing section 8, or where a census of the population is carried out or by an Act of the National Assembly.)
Where the boundaries of any Senatorial district or Federal constituency established under section 71 of this Constitution are altered in accordance with the provisions section 73 hereof, the alteration shall come into effect after it has been approved by each House of the National Assembly and after the current life of the Senate (in the case of an alteration to the boundaries of a Senatorial district) or the House of Representatives (in the case of an alteration to the boundaries of a Federal constituency).
(This above provision means that regardless of when boundaries of any Federal constituency or Senatorial district is altered, such alteration made by INEC can only come into effect when each house of the National Assembly agrees to it and when the subsequent members of each house (i.e. the Senate in situations where an alteration was made to a Senatorial district and the House of Representatives where an alteration was made to a Federal constituency) has completed its tenure.)