changing the instruments within a score

When you add a piano to the score, by default it has two staves. You can't then change just one stave to a different instrument. You could, for example, change to another two-stave instrument, like harp perhaps.

It is not exactly clear what you are trying to do, but I am guessing you want an initial section with Piano + solo soprano followed by a two stave section for soprano+alto on the upper stave and tenor+bass on the lower stave One way to get this would be to start with the SA/TB template and add a piano and a soli soprano from the instrument panel: see https://musescore.org/en/handbook/4/working-instruments#add-instruments
For the piano and solo soprano section, enter them on their respective staved leaving the SA TB staves empty. Then enter SATB on their respective staves leaving the piano and solo soprano staves empty in that section. Then set the staves to be hidden when empty. See https://musescore.org/en/handbook/4/showing-staves-only-where-needed.

In other words start with all the staves you are going to need but show them only when needed.