What comes to mind:

1) Selection Tool.

Select the region that should be changed.

2) Utilities menu > Change Instrument…

Have you tried that?

Yes, I have tried it and it doesn't solve my problem. When I do that, the only thing that changes is the perc clef to a treble clef, but I need a grand staff (both treble and bass clef)

What works for me:

1) Use the Utilities menu, and change the instrument to Piano.

This step will give you Grand Staff.

2) Select the Grand Staff (= both staves!), and use the Utilities menu to change the instrument to Marimba.

In this case the Grand Staff will remain.

Interesting. I'll give it a try. Thank you Peter

Dear Peter,

I have a similar situation.  I am writing a solo Guzheng piece.  Finale defaults to a single staff for this instrument, but I want to use a grand staff to notate the piece.  Weirdly, when I use Utilities>Change Instrument> there is NO PIANO available!  Why would this be?  

Under keyboards, there is only Melodica, Synth Bass, Synth Soundtrack, Sound FX, Ondes Martenot, and Theremin.  There is no option under Black Staff to select a grand staff either.  Can you explain why this might be? 

How should I change from single staff to grand staff?  Or should I copy everything I've already done, delete the instrument and start again with a piano?

Hello George,

Just an idea (it just might work):

Make the change in two steps.

First, change to a completely different instrument (e. g. a Tuba).

Next, change the Tuba to a Piano.

If that does not work, try creating a fresh, new document with Piano, and take it from there.