Transition

I think the most difficult is going from 2 stroke to 4 stroke. Out of James, RC, and RV, I think James' transition was the most difficult. He went from 125 2 stroke, had about 6 months on a 250 2 stroke, and then went right into a 450 4 stroke racing against RC. RV will have an easier time transitioning. he is going from 4 stroke to 4 stroke.

But, to answer your question. I think it is easier going from 250 4 stroke to a 450 4 stroke.

Obviously its easier going from a 250 four stroke to a 450. Back in the day, riders struggled a little bit adapting to the power of a 250 two-stroke when moving up from the 125's. These days it seems the kids have no problems.

125-250 is way harder than 250f-450f.
Way easier to hop on 4 strokes if you spend time of 2's than the other way around.