Transposing down an octave on grand staff

Copy and paste, see: https://musescore.org/en/handbook/transposition#manual-transpose ?

If absolutely everything below middle C is going onto the Bass clef, then you can use the Split staff... command that's found when you right click the staff. This will split the notes for you. You can then Select the staff that will be be created and the original bass clef staff and implode them Tools->implode (Edit->Tools->Implode in version 2). to combine them on a single staff. You can then open the Instruments dialog by pressing i and removing the extra staff from the bottom of the instrument.

If there are the occasional notes you want move between the staves at this point, you will need to delete the note in one staff and enter it in the other, but it will be far less work at this point.