The Legislature shall redistrict the state after each federal decennial census. Such districts shall not be changed except upon the concurrence of a majority of the members of the Legislature. In any such redistricting, county lines shall be followed whenever practicable, but other established lines may be followed at the discretion of the Legislature. Whenever the state is so redistricted the members elected prior to the redistricting shall continue in office, and the law providing for such redistricting shall where necessary specify the newly established district which they shall represent for the balance of their term.