adding instruments

I used strange voices in the above. I now want to rename them as Sop, Sop, Alto, Tenor, Bass. Can I do that directly? Or do I have to add five voices with the real names and copy notes from boy sop to sop1, mezzo sop to sop2 and so on.I tried to Edit Instrument and add five new voices below with Sop, Sop, Alto, Tenor, Bass/ Add/ OK but this did not work. There is also a big gap on page 4 (I think) where you would think that a system should go but Muse Score takes a new page.