If I change the default user cover photo in site settings, will it change everyone's banner on their profile?

This would depend on what settings were turned on at the point of changing the default profile banner image within site settings. If you add a new cover photo into the site settings then this would behave one of two ways, depending on whether you allow users to change their own cover photos or not. If you have allowed users to change their own cover photos, then this would not overwrite their choices on their profiles if they have already set them, whereas any users that had not changed their banner image from the default would see their cover photo changed. Users can be allowed to add their own cover photos to their own profiles by sliding the ‘User Cover Photo’ setting to ‘On', which can be found directly above the default cover photo image within site settings.