- If you replace a belt with a belt with a different curvature (right angle vs. straight), you pick up the items on it.
- If you replace a belt with a belt with the same curvature (reverse direction or opposite bend), the items stay on it.
- If you rotate a belt with the rotate key, the items stay on it always.
When I'm rerouting belts, it's generally more convenient to use replacement all the time than to press Q and R-several-times when there's an existing belt, but I then end up with extra ore (or circuits or whatever) in my inventory, which I have to pass to logistic trash (or put away by hand) unless the belt has room on it. Since it is possible to rotate belts without clearing them, I wish it could be done via replacement.