varying the amount of staves in a score

hello
i am writing a piece that i have formatted with ie 8 staves for my score.
i have already notated much of the piece, but realise that for the first say 25 bars i only need 2 staves. and later in the piece i might need more staves etc
i don't seem to be able to take out unnecessary staves once i have formatted my score. so it is taking up many pages!
is there a way to vary the amount of staves, or do they remain set once you have set it up at the beginning?

thanks!