Right: time for a bit of controversy here. I do not think that when a two-stroke engine switches from four-stroking to two-stroking, the 'switch' has much, if anything, to do with air density or the effect of G on the fuel. I think, from my experience with motorcycles as well as control-line model aircraft, that the chief factor is LOAD. The simplest way of demonstrating this with a model aero engine is to bench-mount it - I used an old Merco .29 - and run it on a largish prop. (I used a Tornado 10x6) with the needle set to a two-stroke but not leaned right out. Stop it without touching the needle, and change the prop. to one that imposes rather less load (a Master 9x4 in my case). Re-start, and you will find that the engine four-strokes: the needle will have to be closed somewhat to make it two-stroke - at, of course, a much higher speed than with the larger prop..
Alternatively, ride a two-stroke motorcycle in a low gear on a level road, on a light throttle so that it four-strokes. Without moving the throttle, head up a slope: you will hear the engine 'switch' to a two-stroke.
My belief is therefore that the 'switch' is triggered by the extra load imposed on the engine by, typically, pulling the model from low-level flight into a climb. I've noticed, with some model/engine combinations, that in a dive from overhead, the engine will slip back into a four-stroke BEFORE you pull out into low-level flight.
One final thing: as a musician (I'm a blues and ragtime guitar picker), I have a fairly good sense of pitch, and I am not convinced that the increase in RPM when the switch occurs is all that great. On the mororcycle, in fact, it doesn't alter at all.
There: that's my 'two penn'orth', as we say in England. It's based solely on my own observation and the above-mentioned experiments, and I shall be very interested to hear what those more expert than me hve to say about it.
John