Practically speaking, most enterprises migrating to a software-defined network (SDN) must do so incrementally. Panopticon offers an approach for designing and operating an interim hybrid network that combines both traditional and SDN switches by exposing a logical SDN abstraction.
Canini, M., Feldmann, A., Levin, D., Schaffert, F., & Schmid, S. (2014). Software-Defined Networks: Incremental Deployment with Panopticon. Computer (New York), 47(11), 56-60. https://doi.org/10.1109/MC.2014.330 (Original work published 2014)