Any district choosing to operate under this chapter shall not use the processes provided for raising revenue under any other law: PROVIDED, That if for any reason it is deemed more just and advisable by the board, any such other method or process for raising revenue as provided by law may be used concurrently against properties solely within the territorial limits of the district for the sole purpose of extinguishing indebtedness incurred before the district adopts the procedure of this chapter, in which event no funds raised under this chapter shall be used to pay such prior indebtedness. However, when a drainage district issues special assessment bonds or notes after June 1, 1986, the process of raising revenue related to the bonds or notes shall be as specified in chapter
85.38 RCW.