Transitions that go from one scene to the other with nothing in between: crossfade, push, crossWipe, crossIris and crossDissolve, here with a clock face that sweeps like a hand. Both rooms are on screen at once: the one going out as its last picture, still, and the one coming in already playing.
A transition of your own does the same with cross: true, reading the old room with sampleOutgoing(uv).
Click the preview first so it hears the keyboard. 1 to 5 choose the transition, E changes the curve, X goes. M plays a .motion file that changes room the same way, written as data.