How to move notes from one staff to another
I have a piano staff and I want some notes from the upper system to appear in the lower system. I have done that before but can't remember how to do it. I tried lots of searches in the handbook like "move notes to lower staff" etc but none of the matches are the right ones.
Cross staff notation is the way indeed f that's the result you actually want - the notes "living" on one staff but "temporarily visiting" the other. Usually if you have a beamed group that crosses staves, or for something like a three-part fugue where the middle voice wanders from staff to staff.
Otherwise, though, if you're looking for a more "permanent" change of address for those notes, cut & paste is the way.
If you need further assistance, please attach your score and describe what you are trying to do in more detail.