Changing time signatures and key signatures - across pages

Courtesy signatures can get switched off

Courtesy time and key signatures are normal at the end of a system no matter if it is the last one on the page or somewhere else. Musicians expect to see this and may actually not notice a key change if this is missing. I saw this happen not too long ago because it was missing from the printed score.

There are exceptions to this. If there are multiple movements in a song, the is no courtesy key or time signature at the end of a movement. The double barline at the end of the movement and some sort of title telling the musician that a new movement is starting is the key to look closely at the key and time signature just as if it were a new song. This is true even if there is an "Attaca" at the end of the movement. "Attaca" means to play the next movement without a pause.

The other exception is in an exercise book. Courtesy accidentals are not expected at the end of an exercise since each one is intended as it's own song.

MuseScore has 3 kinds of breaks to help with this. System break, forces the next measure to start on the next system. It will start on the same page if it will fit or the next page if it won't. It will show courtesy signatures unless you have turned them off. These are controlled in the Style settings. Page breaks are identical to system breaks except that the next system always starts on the next page. Section breaks cause the next measure to start on the next system but will never show courtesy clefs. By default it will also do 3 other things. Show the long Instrument name at the start of the next system, Restart measure numbering at 1 and pause for 3 seconds. You can select the system break and adjust these 3 options in the inspector.