The Flight Instructor, Airplane SE Land and Sea Practical Test Standards, Introduction, Aircraft and Equipment required for the Practical Test, states "...A complex airplane must be furnished for the performance of takeoff and landing maneuvers, and appropriate emergency procedures. A complex landplane is one having a retractable gear, flaps, and controllable propeller."
A complex aircraft is required for much of the test. To switch aircraft mid-test just adds unneeded complexity to the ride. And "most" examiners are going to "suggest" you do it in one aircraft. Make them comfortable.