Change staff from treble (sketch) to grand?

I’ve seen a couple of posts asking about the opposite direction, but none about this case. I can change the underlying instrument easily enough, but when I do so, all the notes are moved to the upper of the grand staff. I also tried select all/cut, then paste special/explode, but that doesn’t give me the result I want. Is there a simple way to split them in the conventional manner?

IIRC someone suggested the simplest way is to do a marquee selection on the Key Editor followed by [Edit] alt-M(move).

Not all that simple over 100 measures, but alt-down (for me) transposes down one step. It must be some other combination.

Oops - I meant Alt-M!!
(I’ve edited the original)

Bingo! Thanks.