A method for migrating data traffic from an existing optical WDM transmission system to a new optical WDM transmission system, the existing optical WDM transmission system using a first optical transmission band and the new optical WDM transmission system being capable of using a second optical transmission band, wherein the second optical transmission band at least partially comprises the first optical transmission band and a further extension band that does not overlap with the first optical transmission band, the method comprising the steps of: (a) setting up, at each network node location, a network node of the new optical WDM transmission system in parallel to a respective network node of the existing optical WDM transmission system, disconnecting each network interface of the respective network node of the existing optical WDM transmission system from a respective optical network path and connecting each respective optical network path and each respective network interface (102; 202) of the existing and the new optical WDM transmission system to an associated optical migration filter device, (b) wherein each optical migration filter device is adapted (i) to direct incoming data traffic that is received via the respective optical network path and that uses the extension band to the network node of the new optical WDM transmission system, (ii) to direct outgoing data traffic originating from the network node of the new optical WDM transmission system that uses the extension band to the respective optical network path, (iii) to direct incoming data traffic that is received via the respective optical network path and that does not use the extension band to the network node of the existing optical WDM transmission system, and (iv) to direct outgoing data traffic originating from the network node of the existing optical WDM transmission system to the respective optical network path; (c) operating simultaneously, for the duration of a migration phase, the existing and the new optical WDM transmission system, wherein the new optical WDM transmission system uses the extension band only; (d) switching, during the migration phase, all traffic relationships handled by the existing optical WDM transmission system to the new optical WDM transmission system; (e) removing, at each network node location, all optical migration filter devices and connecting all optical network paths to the respective network interfaces of the network node of the new optical WDM transmission system when all traffic relationships handled by the respective network node of the existing system or when all traffic relationships handled by the existing optical WDM transmission system have been switched to the new optical WDM transmission system. (a) setting up, at each network node location, a network node of the new optical WDM transmission system in parallel to a respective network node of the existing optical WDM transmission system, disconnecting each network interface of the respective network node of the existing optical WDM transmission system from a respective optical network path and connecting each respective optical network path and each respective network interface (102; 202) of the existing and the new optical WDM transmission system to an associated optical migration filter device, (b) wherein each optical migration filter device is adapted (i) to direct incoming data traffic that is received via the respective optical network path and that uses the extension band to the network node of the new optical WDM transmission system, (ii) to direct outgoing data traffic originating from the network node of the new optical WDM transmission system that uses the extension band to the respective optical network path, (iii) to direct incoming data traffic that is received via the respective optical network path and that does not use the extension band to the network node of the existing optical WDM transmission system, and (iv) to direct outgoing data traffic originating from the network node of the existing optical WDM transmission system to the respective optical network path; (i) to direct incoming data traffic that is received via the respective optical network path and that uses the extension band to the network node of the new optical WDM transmission system, (ii) to direct outgoing data traffic originating from the network node of the new optical WDM transmission system that uses the extension band to the respective optical network path, (iii) to direct incoming data traffic that is received via the respective optical network path and that does not use the extension band to the network node of the existing optical WDM transmission system, and (iv) to direct outgoing data traffic originating from the network node of the existing optical WDM transmission system to the respective optical network path; (c) operating simultaneously, for the duration of a migration phase, the existing and the new optical WDM transmission system, wherein the new optical WDM transmission system uses the extension band only; (d) switching, during the migration phase, all traffic relationships handled by the existing optical WDM transmission system to the new optical WDM transmission system; (e) removing, at each network node location, all optical migration filter devices and connecting all optical network paths to the respective network interfaces of the network node of the new optical WDM transmission system when all traffic relationships handled by the respective network node of the existing system or when all traffic relationships handled by the existing optical WDM transmission system have been switched to the new optical WDM transmission system.