AI, which isn't guaranteed 100% accurate, offers this...
"Yes, it is possible in NCH Crescendo to create an introduction using both treble and bass clefs, and then switch to bass clef only after the intro.
To start with both clefs (a grand staff): You can create a score with both treble and bass staves, which is standard for piano and similar instruments.
Switching to bass clef only: After your intro, you can remove the treble staff and continue with just the bass staff. To do this, use the "Parts" or "Staves" options to add or remove staves as needed. You can also change the clef on any staff by right-clicking the clef symbol and selecting the desired clef (treble, bass, alto, or tenor).
Steps:
Create your score with both treble and bass clefs for the intro.
After the intro, remove the treble staff so only the bass staff remains, or change the clef of the upper staff to bass if you want to keep two staves but both in bass clef.
To change a clef, right-click on the clef symbol and select the new clef.
Note: Crescendo does not currently support changing clefs mid-measure with small clef symbols (as seen in some advanced scores), but you can change clefs at the start of a measure or remove a staff entirely.
In summary, you can write an intro with both treble and bass clefs and then continue with only the bass clef by removing the treble staff or changing its clef as needed."