NB: The following applies to classic MacOS, ie OS 9.x.x and below. OS X is a little more complex, but the same approximate conditions apply.
The only thing that will change is the contents of the System Folder - and, if they are installed, any Apple-supplied utilities, applications, wizards, etc (such as iTunes, Apple System Profiler, Applescript, Apple CD Player) - ie, anything that was installed at the same time as the system software. These will be upgraded as well if there is an upgrade for them on the installer you are using. They will not be deleted.
It is recommended when upgrading (classic OS) to set Extensions Manager to Apple Default Set (or Apple All? It's been a while...), then restart. That way all the Apple-installed extensions and control panels will be in the right folder to be detected by the upgrader.
Safest way to up- or downgrade is to do an "archive and install" - the existing System Folder will be renamed and put aside so you can manually retrieve things from it afterwards if you need to (such as user-installed extensions and control panels).
The only thing that will change is the contents of the System Folder - and, if they are installed, any Apple-supplied utilities, applications, wizards, etc (such as iTunes, Apple System Profiler, Applescript, Apple CD Player) - ie, anything that was installed at the same time as the system software. These will be upgraded as well if there is an upgrade for them on the installer you are using. They will not be deleted.
It is recommended when upgrading (classic OS) to set Extensions Manager to Apple Default Set (or Apple All? It's been a while...), then restart. That way all the Apple-installed extensions and control panels will be in the right folder to be detected by the upgrader.
Safest way to up- or downgrade is to do an "archive and install" - the existing System Folder will be renamed and put aside so you can manually retrieve things from it afterwards if you need to (such as user-installed extensions and control panels).