With constant traffic growth network providers that operate Dense Wavelength Division Multiplexing networks need to quickly find solutions to upgrade their networks. A key challenge represents coping with increased traffic demands while utilizing minimal resources. Simultaneously, the availability of bandwidth variable transceivers offers network planners the possibility to flexibly allocate resources without excessively overprovisioning the network. However, this advantage increases the complexity of planning algorithms: varying data rate, channel bandwidth, modulation format, and corresponding receiver sensitivity must be accounted for in each transceiver configuration along with the available frequencies. Furthermore, the allocation of flexible grid channels requires the calculation of the non-linear interference penalty whenever channels are added, removed, or reconfigured.