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To reduce the initial introduction cost of fiber Raman amplifiers, a novel in-service wavelength-band upgrade scheme is proposed. In this scheme, new pump lasers are added to an existing Raman amplifier already carrying wavelength-division-multiplexing (WDM) signals, and all the pump laser driving currents are changed synchronously through the transient period, keeping the WDM signal gain unchanged, while increasing the gain in the new band. We experimentally proved the principle of the proposed scheme in both discrete and distributed Raman amplifiers, even with the existence of signal-gain saturation, nonlinear pump interaction, and pump power loss. We also confirmed error-free wavelength-band upgrade in an eight-channel-WDM transmission system.