re-name instruments/parts

HI, from what I read in the manuel I bought, once I have set up an instrument, I can not go back later & change its name? is that correct? I have a piece with mutliple voices (piano score and singers) When there are two singers with same voice- but they have different notes to sing, and I'd like their labels to be different. How can I distinguish them? Like Bass1 and bass2 (or Bass-O or Bass-deH if I want to do it by characters's initials)? Is there any way after I have all ready set up the staves for them to go back and rename?
thank you for any help & suggestions,
gloria