A refrigeration system includes a receiver, a gas bypass valve, a medium temperature subsystem, a first valve system, a second valve system, and a controller. The gas bypass valve is operable to control a pressure of the refrigerant in the receiver. The medium temperature subsystem includes one or more expansion valves, one or more medium temperature evaporators, and a suction group including two or more transcritical compressors operable to compress gas refrigerant and discharge the compressed gas refrigerant into a discharge line. The first valve system is fluidly coupled to a first one of the transcritical compressors. The second valve system is fluidly coupled to a second one of the transcritical compressors. The controller is configured to activate each of the first and second transcritical compressors to operate as a parallel compressor by modulating its valve system to switch a suction input of the transcritical compressor from an evaporator outlet of the one or more medium temperature evaporators to the outlet of the receiver.