Sure! In WDM, Each channel produces their own iformation independent of the other channels. The higher the bit rate, the more the bandwidth nneded for each of these separate channels. Then the channels are multiplexed, with each of them having a carrier frequency (or wavelegth) different than the others (with a spacing so that they won’t overlap in frequency domain) to enter the line. The bandwidth for each channel is still the same as it was when the channels were formed separately and they still maintain their bit rate.
At the receiver, they are demultiplexed (which means their carrier frequency is omitted so that they become the same as they were before multiplexing), so their bandwidth and bit rate remain unchanged. And that’s it! Now if the bandwidth for each channel (including spacing) is, say, 50GHz, the overall bandwidth for the signal after multiplexing and before demultiplexing for a 4 user system is 4 x 50 = 200GHz. But when the signal is demultiplexed, each user again has a signal with bandwidth of 50 GHz. And the bit rate for each channel has not changed.