What changes cause Settings to be erased ?

I'm considering making changes to properties.xml, settings.xml and strings.xml in a project that has a number of users.

I don't want the user's settings to be erased when the app is updated and run.

I'm testing with the simulator and find that even changing the contents of a string in strings.xml causes the settings to be erased.

Does anyone know what changes to these files cause the user's settings to be erased ?