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In this invited contribution, we discuss technology options for bandwidth-variable transceivers which are key components for the realization of flexible software-defined optical networking. Bandwidth-variable transceivers enable the software-controlled adaptation of physical layer parameters such as transmitted bit rate, spectral efficiency and transparent reach according to the traffic demands at hand. In particular, we focus on recent advances in four-dimensional modulation formats and in modulation format transparent data-aided digital signal processing. It is argued that four-dimensional modulation formats present an attractive complement to conventional polarization-multiplexed formats in the context of bandwidth-variable transceivers, where they enable a smooth transition with respect to spectral efficiency while requiring marginal additional hardware effort. Results of numerical simulations and experiments …