Where there's request to change Speed and Duplex Port Ethernet ONT on OLT GPON Huawei MA5603T (for example to 100Mbps), it's actually simple to do. The default port-type is GE (1 Gigabit Ethernet / 1000Mbps):
Huawei_MA5603T#config
Huawei_MA5603T(config)#interface gpon 0/2
Huawei_MA5603T(config-if-gpon-0/2)#ont port attribute 4 5 eth 1 speed 100 duplex full <--- (Port 1 will be change to 100 from 1000)