A sector is one flight or one land segment, so your NRT-ICN and ICN-BKK flights are two sectors. In order to change flights for free before departure, you have to keep the same routing NRT-ICN-BKK but you can change airline and date. If you switch to the direct flight NRT-BKK or change the ICN connection to OSA, this is considered a rerouting and would require the $125 rerouting fee.