well its not strictly NECESSARY. the ECU will detect the difference and slowly adjust until it "learns" the new condition. the problem is this will take several days and you won't feel an immediate gain or the motor might run rough for a while. you can reset the ECU one of 2 ways.
1) disconnect the battery for about 30 minutes
2) there's 4 plugs under the steering wheel...there's a proper way of connecting them and starting the car and driving it a few feet...I don't know exacly how but I think there's instructions on scoobymods.